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 



I'm wondering if this is a problem with Entity::scene not being set yet.  The scene doesn't get set on the entity until the scene sees the entity added.  Creating the object this way, maybe that isn't set.