Multiple fixtures attached to a single body

[sntran]

Need to support adding more fixtures to the body, probably through calling chain, such as

Crafty.e("Box2D").rectangle(w, h, x, y).circle(radius, x, y).polygon([x1, y1], [x2, y2], [x3, y3])

The question(s) is: does the new fixture/shape need to overlap the current fixture/shape? Otherwise, we would have a really big MBR with empty space in between. Box2D would handle it just fine, but for other Crafty's components, it may cause problem.