Fixing several minor issues

Okay, I decided to make a branch off your master, and send along these four minor (but important, for me ;-) fixes.

- #69: Allow destroy() to be called by user without generating messages
- #66: Remove warning in ofxBox2dBaseShape::setData()
- #65: Polygon create doesn't like 3-sided polygons
- #64: ofxBox2dPolygon::clear() calls itself