Updated to Box2D v.2.3, improved information about how to build Box2D in the README.

Box2D 2.3 has very few API differences from 2.1, so the changes are pretty minor when
going between the two. Mostly just adding explicit params to wake up objects when
applying forces.

NOTE: Box2D v.2.3 uses a simple makefile that has no "make install" step. You have to
manually copy the files to locations that are searched by Leges Motus for libs and
includes.