Fix @user mentions

This pull request fixes #106. It does so by replacing slack's `<@U1234>` formatting with the equivalent `@username` before passing the message onto the rest of hubot.  As a result @-mentions work correctly when addressing hubot directly (eg: "@​hubot: ping") but also elsewhere in messages (eg: "@​hubot: what roles does @​ph have").

It fixes half of #9. A similar fix could be applied to the other half easily enough...