More formatting fixes

This pull request (Re)-fixes #9 by implementing almost all of the [Slack text formatting spec](https://api.slack.com/docs/formatting). When receiving messages we'll convert Slack's `<!everyone>`, `<#C1234>` and `<@U1234>` formatting into the equivalent text before passing it onto the rest of hubot.


Should this assertion still pass?  If so it seems worth leaving it in.  As far as I can tell this is the only place `<@U\d+>` is tested.