Merged and fixed conflicts with latest master.
Fixes to RigidFrame, Body, and FixedFrame that remove bugs for instantiating from xml.
Improved caching in FixedFrame for better computational efficiency.  During addToSystem, the FixedFrames traverse their connection trees to cache their anchor body and the transform between the anchor body and itself.  This cache and the system are invalidated if the frame's transform or parent frame change.