Refactored to support qml-box2d's transition to qboject based bodies

Re-factored to support qml-box2d's transition to qboject based bodies and target properties.  Added PhysicsEntity and some Box2D Body/Fixture convenience components.  Entity and Sprite no longer inherit from Box2DBody.  Fixes issue #67 

Fixed installation of plugin.types issue #83 
Fixed Scene.debug handling, fixes issue #59 
Log warnings/errors from ScriptBehavior, fixes issue #25