Include Sentry as an available dispatcher

Sentry acts a little differently than the other dispatchers because you can provide custom information to the log.  I'm providing hopefully a clear enough comment to explain.

Due to the nature of statsd I can see the sentry project dashboard becoming extremely noisy unless a mechanism to stack events by `event.name` is also made available.  Maybe this is in the scope of a future release?  Your call.

Thoughts?
Move this config back under the `slack` property