Add Box2D closest raycast support

Adding closest raycast support, as taken from the following thread/post:
- http://getmoai.com/forums/raycast-for-box2d-hack-t562/
- http://www.fivevolthigh.com/2013/10/moai-exposing-box2d-world-ray-casting-to-moai/
This feature is working well and complete in its own right but Box2D does have additional raycasting functionality that should be exposed too, via a callback to test if raycast should continue on through a fixture or not.  However even without that callback this closest raycast is still extremely useful.