fix fixture id memory leaks issue

when retrieve id property from fixture, I retain the id(which is a CCString obj).
so I add a fixtureIdArray to store these fixture ids,
then whenever you restart game and call addShapesWithFile method, it will call reset() method.

I do the clean fixture id string action in reset method.

why will I add the reset method at the beginning of the addShapesWithFile() method.
because I observe that when you restart your game , and the second time your call addShapesWithFile method, it will cause memory leaks.