Moved /multicast/${group} to a dynamic node structure

refs #145 

moved the multicast group registration in names.api to dynamic nodes. I.e. `/multicast/data.api` is dynamic and is of `application/vnd.ozp-iwc-address-list-v1+json` contentType.

Registrations on each  multicast group will follow the `/` path style. I.e `/multicast/data.api/asdf.1234` is an `application/vnd.ozp-iwc-address-v1+json` resource containing heartbeat information about the multicastParticipant.