This simplifies testing stateful bots by integrating the StateHandler
into the test library. As a side-effect, the mock bot handler gets
reused during a test, making the tests more realistic. The
StateHandler now keeps its state during a call to check_expected_responses,
forcing some stateful tests to be more verbose and explicit.
doc.md better describes the style of documentation that will live
inside these files, since we want these to be similar to our
webhooks' doc.md files in terms of how these are rendered and
composed of Markdown macros.
