added basic etcd support

Hi
I've implemented etcd support for goreman. The short history is that I've used foreman and honcho with an external env repository support. It was very rudimentar but worked for me. A couple of weeks ago I've found etcd and goreman, and as I'm switching my codebase to Go it made sense to try my hand on on integrating both projects. I don't know if this is something that you were planning or if you feel it's right, but there it goes a pull request.
I will keep working on it to make more complete. As for now it introduces a etcd keyword to the Procfile. The string passed will be used as keyspace for etcd as http://localhost/v1/keys/<appname>/ that should contain app data to be passed as environment vars dynamically. This way if you change values on etcd, the next instance of goreman will use the newer data.


Thanks