Modify WebSocket continuation support to not expose Continuations at Message-level

Right now the WebSocket implementation, while it does support continuation frames, does nothing to combine those frames together into single messages, making it somewhat difficult to use the library. (This is especially important because, as noted [in the former WS continuation issue](https://github.com/actix/actix-web/issues/916), Chrome uses continuations all the time.)

I think the intent of the WebSocket RFC is for you to not need to worry about continuations at the message level, so the `Message` struct probably shouldn't include them.