Use code coverage to identify, address testing gaps

[falquaddoomi]

As @d33bs mentioned in #60 (comment), our code coverage in specific modules (e.g., models.py) is low. While I would recommend against writing tests simply to increase code coverage, I agree that the lack of coverage indicate parts of the code that are untested, and if they intersect important functionality then we should definitely write tests for that functionality.

At a high level, I think we should:

1. add coverage reporting to our automated testing suite
2. identify and prioritize functionality that's uncovered and thus untested
3. start adding tests for that functionality in order of priority