Design unit tests

[ryanjgallagher]

There are no unit tests for the code right now. It's mostly tested by checking to make sure the code still runs after making changes, and making sure the results are consistent with previous versions.

Obviously this is problematic. There are a lot of moving parts. Unit tests should focus particularly on making sure all of the word contribution components are correct, and that they're being manipulated correctly into different bars for the word shift graph.