Set b2JointDef::bodyA/bodyB in Box2DJoint

Avoiding some more duplicated code.
Some fixture cleanup and fixes

* Re-create Polygon, Chain and Edge fixtures when their vertices change
* Re-create Chain when loop, prevVertex or nextVertex are changed
* Initialize mPrevVertexFlag and mNextVertexFlag to false
* Removed the 'factorWidth' and 'factorHeight' functionality
* Removed the 'Box2DVerticesShape' base class
Added Body.applyForceToCenter
Small improvement to fixtures example

Keeping the circle centered when changing its radius.
Merge pull request #38 from bjorn/master

Somewhat random list of changes