Fix TrackingCodes update and Get User/Group

The TrackingCode array was serialised incorrectly to Map instead of
List and the items had different value then expected. It's fixed and
also the BoxUser is updated to match its value type.

Two new method was added to get User/Group by ID.
Fix TrackingCodes update test

The test file contained wrong representation of tracking_codes which
caused the unit test fail.
Merge pull request #204 from laszlohordos/master

Fix TrackingCodes update and add Get User/Group method