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

The position= and position() method were incorrect when the body was not set (or after the body had been removed). Basically if body is not set, a PhysicsSprite behaves as a regular sprite now.


It's not super important, but I needed this in one of my games. At the end of a physics animation, I wanted to remove the PhysicsSprite from the screen, without disturbing the other box2d object. So I removed the body from the PhysicsSprite and started applying some cocos2d animations (scale.by and move.to) .Scale.by was working but move.to wasn't…


ps: you might received a similar pull request from me a few minutes ago, but I messed things up and had a commit from another pulled request mixed in... I delete the previous branch and recreated this branch with only this one commit (not sure if github recalled the previous pullrequest though...)