Fix for issue #917, Physics.P2.Body#addToWorld

Fix for issue #917: Calling reset() on Sprite with a P2 body can result in body.data.world == null.
Calling addToWorld() would previously not check the _toRemove array, which could, if the timing were right, result in a Sprite being revived but then removed from the P2 World--the result of this being the Sprite's data would be in a mixed state causing it to appear visually but not function in the world.
Merge pull request #925 from jonkelling/dev

Fix for issue #917, Physics.P2.Body#addToWorld