Python records where OpenMM is installed, so it doesn't need to be specified with environment variables at runtime

The goal is to make installing and using OpenMM easier.  setup.py always knows where OpenMM is installed (it requires this to be specified by setting environment variables before running it), so it now records that information.  This means that 1) you don't have to add the OpenMM lib directory to your library path, and 2) you don't have to set OPENMM_PLUGIN_DIR if you're installing in a nonstandard location.  The former took some strange hacks on Macs, since distutil's runtime_library_dirs doesn't work correctly there.

None of this will apply to Windows, since we use binary installers there.  Any suggestions on how I could improve the behavior there?