Changed the message response design to allow more than one response
to a single message. This enables progress updates or repeated status
updates for long tasks triggered by bot action. It also allows me to
dodge the question of callbacks or promises in the API, because responses
are now sent by calling `send()` on a message object instead of firing
off a single asynchronous response.

Updated all tests to match.

Updated the readme.

Added sinon to the dev deps because I anticipate using it for mocking
service apis. Maybe.