Add automatic token refreshment

[jackleland]

Adds functionality for automatically refreshing tokens when the one used is returned via a 401. Stops the workflow and asks for a username and password, which will kill automatic workflows but going to a access+refresh token situation didn't seem like a better option – I have added a flag for turning off auto-refresh onto the client constructor so this can be deactivated if necessary! The refreshment automatically updates the config file with the new token, I'm not 100% how this would work with mulitple clients open at teh same time though - we might want the auto-refresh to first check the config file to see if the token has been updated and then do a refreshment call? Will have a think abou this...

I've also added a method on the client (update_token()) so that users can manually refresh their token without intervention.

These changes necessitated a bit of a refactor, so 'authentication.py' and 'config.py' now handle the token and the config file respectively. Have a look and let me know if you think it all makes sense, very happy to go through it in person of course!

[jackleland]

Just pushed a small change and a bug-fix: the bug-fix was missing kwargs on the client child classes, the change was refactoring of the update_token method, making instead a distinction between 'refreshing' the token (i.e. sending a request to the auth-token endpoint with a username and password) and 'rereading' the token which involves literally rereading the config file and remaking the Token within the client. The re-reading only happens a single time upon an 'invalid token' response from the API, so we can now have multiple clients on the go at once.