Allow to map Web Socket directly without invoking WebSocketTransformer explicitly

[tomyeh]

If the URI starts with ws:, the server shall upgrade the request to an instance of WebSocket (dart:io) automatically. For example,

new StreamServer(uriMapping: {
  socket.listen((evt) {
    socket.add("Server received: $evt");
  });
  return socket.done;
}).start();

[tomyeh]

Ready in 5eed2e4