Update require('hubot') to also export Message in addition to its subclasses

This implements the feature request in https://github.com/github/hubot/issues/834 :

> Hubot exports all of the Message subclasses, but it doesn't export Message itself. It should be exported to allow adapters to create new message types that scripts could look for (e.g. a message that represents some event Hubot doesn't model).

cc @kballard https://github.com/slackhq/hubot-slack/pull/133