Migrate to stageit api

Script is migrated to use stageit backend

Notes:

• GraphQL is used for reading data
• Rest API is used for writing data
• Both api's use the same REST api token
• Script works with Python2, there is a dockerfile for testing compatability
• There is some test coverage on most used functions
• Some functions are made deprecated, because stageit does not support them
• Script is now 'lazy loading', it does not parse credentials + construct backend connection until a function is actually called
• README is updated with instructions