Data output agent

This pull request adds a new `DataOutputAgent` that returns a RSS or JSON feed of incoming events.  In order to do this, the EventHooksController will now accept all HTTP verbs, not just POSTs.  (Perhaps the controller should now be renamed to WebRequestsController, or similar, since webhooks are traditionally limited to POST requests?)

This is a solution to the output half of #11.  Appreciate feedback @albertsun, @alias1, @Fluffums, and @snicker!