Clean up ChopGrammar and add tests

[gtback]

@Mraoul : I made the following changes in this branch:

• Tried to make pep8 happy and pylint as happy as possible (pylint can be a little picky)
• Add some basic tests based off the examples provided in the documentation.

There should be no changes to any behavior.

Let me know what you think. You can run the tests yourself by installing pytest (pip install pytest or pip install -r dev-requirements.txt) and running py.test from the root of the repository. I can set up Travis-CI to run tests on every push, if you'd like.

[gtback]

@Mraoul: I added tests for all of the option parsing. The entire ChopGrammar.py file is now covered by tests, except for:

• a single line, representing an error I can't find a way to induce.
• all of the (get|print)_(tree|family) functions,