Simplify importing data container classes

Added get_last_pp_dev_action() and LastPPAction type

New functions: get_users(), find_users(), find_user()

Supports lists in AXI responses

Fix logic mistake in set_subscription()

Finished refactoring for Python 3

[retroactive non-author tag] Data classes for devices and users

[retroactive non-author tag] Implemented create user

[retroactive non-author tag] Initial implementation and docs