[COINBASEPRO-4] feature/ WebsocketClient

[drewrice2]

Issue link: #4

While this is just a first draft, I think it covers most of the functionality from the GDAX Python module's WebsocketClient (to which i was also a contributor).

This implementation purposefully does not deserialize Coinbase's returned JSON. It simply appends the JSON string to a logfile, delimited by newlines.

Please let me know what else you think this needs.

If anyone's interested, the "[COINBASEPRO-1]" tag is a reference to the repository and issue number.

[drewrice2]

@acontry I'll squash-and-merge, should you approve

[acontry]

I appreciate all the work! I want to think carefully about the websocket interface, and also how we can have both threaded and asyncio versions available. I want to wrap up some changes to REST interface then tackle this next.

[drewrice2]

@acontry totally understand. Happy to work together on that if you'd like. Standing by for further instruction.