Added a fire rate to the weapon class.

Fire rate is currently specified in shots per second. It may not be
entirely accurate to the nano second due to both the delay of the Update
cycle and the possibility of floating point error, but for the most part,
normal humans should not notice this nanosecond scale error.
Added an ammo system to the weapon.

Has a total ammo property and a current ammo property to keep track of
ammo.
Added an ammo cheat.

Just press all WASD at the same time to instantly refill your ammo.