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)
I think the code below would be more simple if you use did this:

type clientKey struct {
  clientId string
  roomId string
}

data map[clientKey]clientMsgs

That way, every place where you do a look up, you can do one lookup instead of two.