* References to bodies, fixtures, joints defined at creation are maintained by
RubeScene. Helpful for post processing actions and custom property handling.

* Added first pass of custom property handling for bodies. Currently limited to
strings because that's all I need for my game. This was more or less a port of
the Jb2dJson.java file handling.

* Added .dispose() calls after shapes are used in the FixtureSerializer.

* Fixed sensor handling in FixtureSerializer. FixtureDef references the boolean
"isSensor" while the RUBE Json file references it as just "sensor".