all box2d classes refactored from java.util.ArrayList to badlogic Array class

This does change the API slightly, but since box2d already has some API changes from 0.9.8 to 0.9.9 I figured, why not?

- methods that returned ArrayLists, such as body.getFixtureList(), now return Arrays
- method calls to these objects will need to be change from ArrayList API to Array API, i.e.
    - bodyFixtures.size() --> bodyFixures.size
    - bodyFixtures.isEmpty() --> (bodyFixtures.size == 0)

I built gdx-core from this patch and dropped it into my game for testing.  After changing to Array API, I did not have any errors.