User Control System Modified:

Added click to shoot.  Added control schemes:
1)  Default:  cannon follows mouse cursor and fires when mouse button 1
is clicked, and
2)  Secondary controls:  left and right arrows rotate turret with up
arrow and space bar firing the cannon.  (Note:  as such, left and right
arrows no longer move tank, leaving "a" and "d" as the keys for moving
the tank.)