Bullet, add contact callbacks and add GdxCollisionObjectBridge

- Add ContactAddedListener (optimized for ByWrapper, ByObject and ByValue)
- Add ContactProcessedListener (optimized for ByObject and ByValue)
- Add ContactDestroyedListener

With this all collision detections methods are implemented described by: http://bulletphysics.org/mediawiki-1.5.8/index.php/Collision_Callbacks_and_Triggers.
- Add ContactCallbackTest
- Add btManifoldPoint#getUserValue/setUserValue

I noticed that my previous implementation of btCollisionObject#getUserValue/setUserValue might not be future proof. So I also added a GdxCollisionObject which allows for futher extensions, like contact filtering and therefor reducing JNI overhead.