What is claimed is:1. A harrow comprising:an elongate tool bar;a plurality of wheels supporting the tool bar on the ground for movement in a working direction perpendicular to the tool bar; anda plurality of harrow elements mounted on the tool bar to extend generally rearward therefrom for movement therewith in the working direction;each harrow element comprising;a pair of support arms pivotally mounted spaced apart on the tool bar, the support arms extending generally rearward therefrom;a frame pivotally mounted on a rear end of the support arms towards a rearward end of the frame such that the frame is pivotal about an axis extending through the rear ends of the support arms;a hanger connected between a front end of the frame and the support arms for suspending the front end of the frame from the support arms;a plurality of tine support bars mounted on the frame transversely to the working direction, the tine support bars being spaced apart across the frame, each tine support bar having a plurality of tines mounted thereon, the tines being spaced apart along the tine support bars, each tine being mounted at an upper end on the tine support bar to extend generally downward to a ground engaging end, each tine support bar being supported for pivotal movement relative to the frame about a respective tine axis extending longitudinally of the tine support bar;a tine adjustment mechanism connected between the tine support bars such that displacement of the tine adjustment mechanism relative to the frame pivots the tine support bars in unison about the respective tine axes for adjusting an angle of the tines relative to the ground as the tine adjustment mechanism is displaced relative to the frame; anda coupling supporting the frame on the hanger, the coupling being pivotally mounted relative to the frame about a coupling aXis extending through the coupling;the tine adjustment mechanism being connected to the coupling such that the coupling is arranged to be pivoted about the coupling axis as the tine adjustment mechanism is displaced relative to the frame to adjust a height of the front end of the frame relative to the support arms in response to adjusting the angle of the tines.2. The harrow according to claim 1 wherein the tine adjustment mechanism comprises an actuator arm mounted transversely to the tine support bars on the frame for longitudinal movement relative to the frame and a crank mounted on each tine support bar, each crank being connected to the actuator arm such that longitudinal displacement of the actuator arm rotates the tine support bars in unison.3. The harrow according to claim 2 wherein there is provided a manual lever mounted on one of the actuator cranks to extend generally upward therefrom for manually adjusting the tine angle.4. The harrow according to claim 2 wherein there is provided a collar mounted on the frame for slidably receiving the actuator arm therethrough, the collar having a number of apertures therein arranged for co-operation with a plurality of apertures in the actuator arm such that a locking member may be inserted therethrough for locking the actuator arm at various longitudinal position corresponding to different tine angles.5. The harrow according to claim 2 wherein there is provided a rocker shaft mounted on the frame generally parallel to the tine support bars for rotation about the coupling axis and a first crank mounted on the rocker shaft for rotation therewith, the first crank being connected to the actuator arm for rotating the rocker shaft with the tine support bars, the coupling comprising at least one second crank, each second crank mounting a support link on a free end thereof, each support link being mounted on one of the support arms such that rotation of the tines in a downward direction will rotate the free end of each second crank downward and raise the front end of the frame relative to the support arms.6. The harrow according to claim 5 wherein there is provided a plurality of mounting locations on the first crank, each mounting location being arranged to mount the actuator arm thereon corresponding to a different height of the front end of the frame relative to the rear end for adjusting fore-aft pressure on the tines.7. The harrow according to claim 5 wherein there is provided a plurality of mounting locations on each second crank, each mounting location being arranged to mount the corresponding support link thereon corresponding to a different height of the front end of the frame relative to the rear end for adjusting fore-aft pressure on the tines.8. The harrow according to claim 1 wherein the frame comprises a pair of rails mounted substantially parallel and spaced apart to extend rearward from respective pivots on the tool bar, the rails being interconnected by at least two cross members extending therebetween, the support arms being mounted on respective rails at a position spaced towards the rear end of the frame.9. The harrow according to claim 1 wherein there is provided spring biasing means mounted between the support arms of each harrow element and the tool bar, the spring biasing means being arranged to urge the harrow elements downward toward the ground.10. The harrow according to claim 9 wherein the spring biasing means comprises a pair of upright plates mounted on the tool bar at a location corresponding to each support arm, the upright plates pivotally mounting the corresponding support arm therebetween towards a bottom end of the plates, the upright plates pivotally mounting a first end of a spring therebetween towards a top end, the spring extending downward and rearward to a second end mounted on the corresponding support arm spaced from the tool bar, the spring being mounted in compression for urging the go responding harrow element downward.11. The harrow according to claim 10 wherein the tool bar is mounted for pivotal movement about a longitudinal axis such that rotation of the tool bar will pivot the top end of the upright plates forward and rearward for respectively decreasing and increasing a force of compression of the spring for varying downward force on the tines.12. A harrow comprising:an elongate tool bar supported for movement along the ground in a forward working direction extending transversely to the tool bar; anda plurality of harrow elements mounted on the tool bar to extend generally rearward therefrom for movement therewith in the forward working direction, each harrow element comprising;a pair of support arms pivotally mounted spaced apart on the tool bar, the support arms extending generally rearward therefrom;a frame pivotally mounted on a rear end of the support arms towards a rearward end of the frame such that the frame is pivotal about a axis extending through the rear ends of the support arms;a plurality of line support bars mounted on the frame transversely to the working direction, the tine support bars being spaced apart across the frame, each tine support bar having a plurality of tines mounted spaced apart along the tine support bars at respective upper ends to extend generally downward to respective ground engaging ends, the tine support bars being supported for pivotal movement relative to the frame for varying a tine angle relative to the around; anda hanger comprising a pair of support links mounted on the front end of the frame, each support link being slid ably mounted at an upper end on a corresponding one of the support arms, the support links having a portion of increased dimension adjacent the upper end thereof arranged to engage the respective support arm for restricting downward movement of the front end of the frame relative to the support arms while permitting the front end to float upward relative to the support arms in response to elevation changes of the ground as the frame is displaced in the working direction.13. A harrow comprising:an elongate tool bar supported for movement along the ground in a forward working direction extending transversely to the tool bar; ana plurality of harrow elements mounted on the tool bar to extend generally rearward therefrom for movement therewith in the forward working direction, each harrow element comprising:a pair of support arms pivotally mounted solaced apart on the tool bar, the support arms each comprising a first portion extending upward and rearward from the tool bar to an upper end and a second portion extending downward and rearward from the upper end of the first portion;a frame pivotally mounted on a rear end of the support arms towards a rearward end of the frame such that the frame is pivotal about an axis extending through-the rear ends of the support arms, a front end of the frame being spaced below the upper end of the support arms such that there is provided a clearance space between the frame and the upper end of the support arms permitting upward deflection of the front end of the frame;a plurality of tine support bars mounted on the frame transversely to the working direction, the tine support bars being spaced apart across the frame, each tine support bar having a plurality of tines mounted spaced apart along the tine support bars at respective upper ends to emend generally downward to respective ground engaging ends, the tine support bars being supported for pivotal movement relative to the frame for varying a tine anal relative to the ground; anda hanger connected between the front end of the frame and the support arms for suspending the front end of the frame from the support arms permitting the front end to float upward relative to the support arms in response to elevation changes of the ground as the frame is displaced in the working direction.