Improve test coverage

[jessegrabowski]

Current testing suite is with in unittest, but it seems pytest is universally preferred among the scientific packages I follow. It will also give access to pytest-cov, which will complement #21

While I'm at it, I would like to add hypothesis as a dependency and re-write tests using that framework. Should ensure more robust coverage of core functionality (especially the parser).

[jessegrabowski]

It turns out pytest and unittest play fine together, and I have pytest-cov set up. Now what is important is to get 100% coverage of the existing code base.