Merge pull request #6 from jchodera/master

Moved tests into separate repex/tests/ directory
Fixed tests to work with new repex organization containing testsystems.
Fixed an undefined global parameter in Diatom that caused an error when use_central_potential=True
Added AlanineDipeptideVacuum and fixed issues with how constraints were specified for AlanineDipeptide* systems.  Note that arguments have now changed for these systems.
Updated custom integrator test.