Import box2d.js

https://github.com/kripken/box2d.js

SHA1: 5c49ced295f2876855965e36951d1097c39060fb
Implement bench2d_asmjs.js to test box2d.js

bench with:
d8 box2d.js/box2d.js bench2d_asmjs.js bench2d_run.js