Small fixes:
    - Adjust CharmmPsfFile parser to accept multiple blank lines before the
      NATOM record (I saw 1 PSF file like this... not sure how common it really
      is)
    - Removed the shebang line from element.py and removed execute permissions--
      it should never be executed directly (evidenced by the fact that the
      original shebang line was wrong and would never work, anyway).
    - Fix a test to determine if a Bond is part of an Angle in the PSF data
      structures.
Fix a bug in which the switching function would not get toggled if requested via
the switchDistance argument to CharmmPsfFile.createSystem().

Also add a check to protect against negative switching distances.
Merge pull request #507 from swails/minor_fixes

Small fixes: