@ashfurrow => add support for authenticating via Xauth

Splits up the ArtsyAPI into separate files.

Creates a new Moya Provider that will add the correct auth details, it takes a UserCredentials struct that contains the access token and the User. I'd expect we'd have both post login via PIN or Email:Password and to use that object to finished the bid fulfilment. 

Tests crash differently each time with the provider. I gave up and just outright blame swift, not wasting more time on it now.