Review vessel uri definition, allow for uuid, mmsi, url

[rob42]

See SignalK/freeboard-sk#22
Server needs to honor the mmsi (and url) names same as uuid

[sbender9]

Yeah, I'm a bit confused now. Seems like things are not how they should be with the servers and the spec seems inconsistent.

/self/mmsi should obviously return the mmsi: 1233455

So then shouldn't /self/uuid be just the uuid: b7590868-1d62-47d9-989c-32321b349fb9?