Add websockets optional dep, and facade wrapper that uses it.

[JamesParrott]

Minimal example of using the standard external dep to implement websockets support.

With a little extra code to parse the scheme, this would facilitate issue #886 (auto following of HTTP redirects) and if a default transport was added, the scheme in the url could determine the transport.

TODO:

• Make the API surface the same, especially the exceptions.
• Benchmark it to make sure any performance penalty isn't too bad
• Enumerate all ways in which Paho's Mock websockets code is not regarded as compliant by the library.