Websocket open callback.

We can now pass some parameters when upgrading to websockets.

    {websockets, CallbackMode, Opts, Params}

When the connection is upgraded, the websocket module now call:

    CallbackMode:handle_message(open, Params)

Which allows you to keep some parameters from the initial request.
On the open callback, you can also send date over the connection.

    handle_message(open, _Params) ->
        yaws_api:websocket_send(self(), {text, <<"Hello world">>}).

TODO:

  * handle compatibility with the previous implementation
  * should we handle return value? Like {close, Reason}?
  * update doc
  * add tests
  * gather feedback

It's just to start a discussion about #98.