Copied parse_set tests to test_parser

Copied for now since there's only one and I'd like to keep the doctests
having a usage example. Also I made it so that the unittests call the
parser.py's doctests to eventually have all the tests runnable with one
command. Also I actually added test_parser.py to the git repository,
which I forgot last time... The perils of late-night commits.
Changed the _RANGE constants to an Enum

Have I mentioned that I love the recent additions to the python standard
library? I think they've done a great job with them. When I deploy them
to PyPI I might want to change this to just a class for compatability
with older versions of Python, unfortunately.