Client side: worker registration and connection to debusine

The worker is a Python client (no Django dependencies) that registers if needed and connects to debusine server awaiting for tasks.

In different discussions (in #4 (closed), !17 (merged)) we seem to have settled on the following approach (please comment if not agreeing or if I've missed anything):

• If the worker configuration doesn't have a token: worker will create a token and register (REST API) to debusine
• Worker will connect using websockets to debusine and wait for instructions (ping, dynamic state, task to execute, etc.)

Using aiohttp for the HTTP REST / websockets.

In this issue the worker will register and connect.

This is part of the user story #4 (closed)