Fix reading in an Amber prmtop file as a topology when it has an ATOMIC_NUMBER
section:

/.../mdtraj/formats/prmtop.pyc
in load_prmtop(filename)
    200         if 'ATOMIC_NUMBER' in raw_data:
    201             try:
--> 202                 element = elem.Element.get_by_atomic_number(int(raw_data['ATOMIC_NUMBER'][index]))
    203             except KeyError:
    204                 element = None

AttributeError: type object 'Element' has no attribute 'get_by_atomic_number'
Recognize .parm7 as an Amber prmtop file.  This is the file extension used
typically by Amber programs as well as VMD.  Makes sense to recognize the
extension here as well.
Merge pull request #429 from swails/master

Fix reading in an Amber prmtop file as a topology when it has an ATOMIC_NUMBER section
Merge pull request #430 from swails/parmext

Associate .parm7 extension with Amber prmtop
Merge remote-tracking branch 'origin/master' into fixmdcrd