A few things:

- Add a PSF parser to instantiate a md.Topology instance from a CHARMM, XPLOR,
  or VMD PSF file

- Translate the recent fix to OpenMM's unit package (regarding the need for a
  deep copy in some instances) to the unit package here

- Add .psf extension recognition for topology files

- A few minor fixes and cleanups for issues flagged by pyflakes (including what
  would be a fatal error when warning about not using extension .lh5).
Add a test for PSF file parsing.
Fix unit handling, small problem with PSF file parsing, and add missing test
file.  Test passes on my machine.
Update attribution.
Add test against VMD to PR
Merge pull request #1 from SimTk/swails-psf

Add test against VMD to PR
Update attribution.
Working for python 2.6?
typo
Merge pull request #2 from SimTk/swails-psf

Python 2.6 support
Merge pull request #585 from swails/psf

PSF file support