Replace mocha and chai with Node.js test runner and assertions

I'd like to use this project to experiment with transitioning from mocha and chai with the Node.js test runner and assertions. The Node.js test runner is very closely modeled after mocha (including support for .only), so it should be a relatively smooth transition in that regard. Using Node.js assertions will require a bigger change since they don't use the fluent API from chai. However, with an expect helper in the test harness, we could probably remap most of the calls.