Added support for Desmond Trajectory family.

First attempt to support Desmond trajectory family. When I try to run nosetests for the whole test suite, I ran into a weird issue with below message:
Fatal Python error: PyThreadState_Get: no current thread
Abort trap: 6
Probably due to some messy dynamic linking issues. In any cases, I am trying my best to test the unit test for dtr module, and it does pass the test.
I think from https://github.com/SimTk/mdtraj/pull/624/files#diff-8f1c3881dd0aa7d49d7a284aef8745a9R46, DESRES_WIN32 is already defined though.