Authenticator updates

[diocas]

Changes in this PR related to the authenticator:

• Easy configuration by using the .wellknown to fetch all the necessary endpoints
• Updated readme and installation (to push this to pypi)
• Automatic refresh of the oAuth tokens
• Configurable way of retrieving the uid of the user and cleaner way of passing it to the spawner
• Access token passed as environment variable to be used in the startup script

Closes #13 Closes #11

[diocas]

@ricastell This means that the first part of the token update is done. Now you will have a token set as env variable, so in the session startup script you will need to save it to the corresponding file so that EOS already works.

Then, inside the container, you need to create a server extension that, from time to time, polls the user information from JH and updates the token file (I would say 1min before the token expires). The following call (inside the user container) will always return a JSON with a valid access token (unless the session expired):

curl -H "Authorization: token $JUPYTERHUB_API_TOKEN" $JPY_HUB_API_URL/users/$USER