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
Nope; we are not leveraging the error-escape mechanism in async.reduce. Instead, each piece of middleware is responsible for catching its own errors and if appropriate, calling `done`. Since async does no error trapping of its own, there is no way `err` could ever be non-null. There is mention of this in the documentation, but I can add code comments as well if you feel that is appropriate.