Improve find simbody

This was originally #12, but I had to create a new PR b/c my fork broke.

I've made 2 changes:
1. remove FindSimbody.cmake, and use Simbody's new SimbodyConfig.cmake script.
2. allow OpenSim to specify the version of Simbody it needs. This is important because we have had people try to use OpenSim with a Simbody that is too new (trying to use OpenSim 3.2 with Simbody 3.4).

No users should feel the difference of this change. I've tested it on Windows and Linux and it should be ready to go in.