Add support for Websocket continuations

As described on the mailing list:

https://groups.google.com/forum/?fromgroups=#!topic/vertx/F6s-eqlNjyM

This adds the option for users to handle the WebSocketFrame in order to handle continuations. It is possible to avoid this by accumulating non final frames in WebSocketImplBase until a final frame is received, I tested this alternate approach and it did work however I was unsure if it is threadsafe to do so. I can pull request the alternative if need be.