Hook up the debug property in Scene to expose a DebugDraw overlay when
physics are enabled.  Toggling debug will now toggle the overlay.

DebugDraw is now an uncreatable type.
Merge pull request #44 from kenvandine/world-in-scene

Automatically add a DebugDraw when debug and physics are both enabled.
Scene Stack with transition animations
add example of scene transition
Merge pull request #53 from paulovap/scene_stack2

Scene Stack with transition animations