Make the event destinations pluggable.

You can now add objects to the #endpoints array that respond to #log_event to send events to other recipients. E.g., another interesting endpoint could be StatsD. It's completely backwards compatible.

I removed the unstructured #log call from the API, because it doesn't really work well in this concept, and I don't think it belongs here in the first place. If you want to log an unstructured message, you'd just use Rails.logger.log.

@fw42 @tobi for review.