a little better version of the struct-b2manifold branch
changed b2FixtureProxy to a struct.
more stability. collision not working. added 'd' key for world dump during run.
looks to be working pretty well. collisions on the left and right side of the world are working.
misc fixes to address some collision dynamics.
Merge pull request #121 from totallyevil/STRUCT_MANIFOLD_R2

Appears to be working very well