A number of ARC improvements:

    - Associate .xyz extension with Tinker XYZ format (ARC)

    - Have Tinker XYZ and ARC files create a topology object to use so one does
      not have to be provided. It still uses a provided topology if provided,
      since XYZ and ARC files have no chain, residue, or element information

    - Add test cases for the ARC loader, ARC with PBC information, and make sure
      that ARC files providing their own topology give the same trajectory
      information as a arc file _not_ providing its own trajectory

    - Fix a bug that was causing Travis-CI to fail.  The ARC-related failure
      should be fixed now.

This should now be ready for merge pending the results of CI.