WebChannel refactor

gistfile1.js

WebChannelBroker is a internal implementation detail of WebChannels
  registerChannel(channel)
  unregisterChannel(channel)

WebChannel
  constructor(channelId, origin) -> WebChannel
  channelId
  origin
  _deliver(message, sender) -> 
  listen(callback)
    this._listenCallback = callback;
    WebChannelBroker.registerChannel(this);
  stopListening()
    this._listenCallback = null;
    WebChannelBroker.unregisterListener(this);
  send(message, [target])


let wc = new WebChannel(channelId, origin)
let cb = function(channelId, message, sender) {
  wc.send({....}, sender);
}

wc.listen(cb)

.......

wc.stopListening()