GitLab

- Make Python 2.7 a minimum requirement per pandegroup/openmm#1656
- Make the bond classes as well as other forcefield.py singletons inherit from
  Singleton to make them pickleable
- Make class Bond inherit from namedtuple instead of tuple

Hard-code CC and CXX to clang for Macs, since gcc/g++ will *not* work with
Anaconda, despite the fact that distutils will try to use them.  System Python,
homebrew, and MacPorts on Macs will always use clang, so this hack should always
work and fix issues with users that have GCC installed from MacPorts or homebrew
*and* Anaconda.

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