Adds sendPong for sending unsolicited pong messages.

According to the websocket specification a pong message can be sent unsolicited. This was not possible with the current API.

Pull requests adds new method sendPong for triggering an unsolicited pong message. Used and tested in an iOS app to keep the connection alive (heartbeat).
nit: add a period. [Comments are sentences][1].

 [1]: http://nedbatchelder.com/blog/201401/comments_should_be_sentences.html