Removed CharacterMovement interface & Added Gravity toggle

Added the Gravity Toggle ability, allowing the player to turn gravity on
and off, and changes their movement to forces, rather than direct
movement.

Removed the CharacterMovement interface.  It was just getting in the way
of using the CharacterMoveScript at this point.  We have no intention of
going back and using the CharacterControllerMovementScript, so both of
these were removed.