Listener Middleware

Builds on #801 (57b722022807b9a5befaafcab2442ad3a03d455b) to allow injecting arbitrary code between the Listener match and execute steps. Like in Express, middleware can interrupt the response process, preventing the Listener callback from ever being executed. Middleware can perform operations both on the way towards the Listener callback (before callback execution) and on the way away from the Listener callback (after callback execution/middleware interrupt).

As a side effect, listeners are now executed asynchronously. Behavior around message.done should remain the same (process until message.done is true).

Example usage (imposes authorization policy on all scripts): https://github.com/michaelansel/hubot-rbac/blob/master/src/rbac.coffee
I think I'd like to see a bit more documentation in this package's README before including it as a fully functional example.  In particular, it should document how to make your own commands compatible with rbac (ie adding metadata) and defining roles for your bot. It also looks like it is a bit verbose, in that it generates a few replies every message regardless of if the listener is blocked.