Add a package to measure test coverage

[kb0rg]

Once we've established a baseline for creating and running tests for Heimdall (epics #99 and #113 ), we should begin to enforce coverage for any new code.

TODO:
Discuss/ clarify expectations here.
(eg: Test coverage reports can be via git hook (maybe?), and merges should be prohibited with missing tests (also maybe).)

[Shadowfiend]

Closing this for now. Someday maybe we want to do this, but it's not at the top of the list.