Add support for sending binary messages.

For issue MostAwesomeDude/txWS#15: adds sending, but not receiving, binary messages.

I needed binary messages in my app and this was the simplest compatible thing that I could think of. Any comments on the API design and code are welcome.

Binary support must be specifically requested via `.setBinaryMode(True)` so as to preserve backwards compatibility with the previous behavior that a str object would be sent as a text frame. I'd be happy to not have such a mode switch, but I didn't want to break existing code.