Merge pull request #388 from peastman/master

Exposed getMolecules() in the public API
Merge pull request #390 from peastman/master

Install SWIG interface definition to the include directory, so outside plugins can use it when building their Python APIs
Merge pull request #391 from peastman/master

Fixed a compilation error in C wrappers
A previous commit had changed OpenMM_headers.i to OpenMMSwigHeaders.i without changing the corresponding include statement in OpenMM.i.
Merge pull request #394 from jchodera/master

Fix broken Python wrappers