Complete the initial draft of the SpecReporter.

I think I already have something I'd gladly use, though it's definitely
not ideal. This closes #2. Reporter code is always ugly, but for some
reason my approach in this library seems a lot messier than the approach
in Mocha. Either way, this is a different language, and these trivial
parts of the code will change radically once we add concurrent test
running.
Test error message propagation in our mock tests.

I should turn these into real effective unit tests.