Physics Comp

The PhysicsComp is now functional.

PhysicsComp is a component which controls the position of a BaseObject,
similar to the RecastAgentComp, which causes the objects position to
mirror that of an invisible btRigidBody (stored and controlled by the
PhysicsHandler) until the body lands on a surface of a given incline or
less.

The PhysicsComp and RecastAgentComp cannot both be active at once, as
both perform the same basic function, so when the PhysicsComp becomes
active it deactivates the RecastAgentComp, and reactivates it once it
settles on a surface.

Also implemented a 'knockback' effect class, which activates the
PhysicsComp.