Updated the RUBE loader w/custom property capabilities and some other fixes

Sensor definitions weren't working.  Fixed.

Added custom property handling.  I've only tested custom properties for Body types, and only the String custom property so far, however.

Don't know if you like singletons or not, but to ease creation I'm using one for RubeScene to make custom properties easier to handle.  I also added references to all bodies, joints, and fixtures that are created by the loader to RubeScene.  I needed this to support my Entity System that post processes all physics objects.

Shapes were not being disposed after use.  I added code to do that.