added game object creation/destruction logging (see #18)
fixed minor reference issue in the simple renderer
a bunch of work on the game state management and physics

see #3.
Simplified GameObjectionCollection and the way that objects refer to
each other.
Switched GameObjects from having members that are box2d objects, to
directly subclassing those objects, in order to avoid all sorts of odd
pointer issues.
Also, apparently you really need to call setPhysics *before* setup for
the box2d objects… otherwise they don’t move at all. It took me way too
long to figure that out.