Switch to Chipmunk for love.physics

Issue #1384 duplicate
coffeecat
created an issue

I don't know whether this has been considered, but Chipmunk (http://chipmunk-physics.net) is now fully open sourced under a MIT license, and performs significantly better than Box2D. (http://chipmunk-physics.net/aboutChipmunk.php)

This could be done at the same time of #1125. Also related: #1365