[W.I.P.] Add service-registry component (feedback wanted).

The goal of this pull request is to add support for registering Respose instances in [Rackspace Service Registry](http://docs.rackspace.com/rsr/api/v1.0/sr-devguide/content/overview.html).

The patch is unfinished and work in progress. All it currently does it adds a filter which performs the following actions:

- Register the instance in service registry in init and start heartbeating
- Stop heartbeating and delete the service on destroy

I opened this pull request to start the discussion and get more feedback from Repose people on how to improve this patch to make it more "idiomatic" and better (best way to add tests, how to correctly use other abstractions such as handler, etc.).

Thanks,
Tomaz