Add a websocket signaling server impl

PTAL
(Sorry for the previous abandoned requests. I'm new to the github work flow and messed up the repository)
Why get the roomID from conns here?  Can't we just have a roomId and clientId in the scope of this function, remembered by the for-loop (gotten from the "register" command)?  Then we don't need a lookup.

Other, perhaps more elegant, option, would be to have rooms.addClient return a *client, and *client could have a method .sendMsgToOtherClientsInRoom.  Then, this would all just be:

client.sendMsgToOtherClientsInRoom(msg.Msg)