fix for position() and position= for PhysicsSprite when body is not set

when body is not set, PhysicsSprite behave as a regular sprite …
The position= and position() method were incorrect when the body was not set (or after the body has been removed). Basically if body is not set, a PhysicsSprite behaves as a regular sprite.
Merge pull request #39 from dprophete/physics_sprite_positon_fix

Fix:
=> Fix for position() and position= for PhysicsSprite when body is not set