Log Message:
Changes 4.0.2:
* Requirements: Now depends on Kombu 4.0.2.
* Tasks: Fixed problem with JSON serialization of group
* Worker: Fixed JSON serialization issue when using inspect active and friends
* App: Fixed saferef errors when using signals
* Prefork: Fixed bug with pack requiring bytes argument on Python 2.7.5 and earlier
* Tasks: Saferepr did not handle unicode in bytestrings on Python 2
* Testing: Added new celery_worker_paremeters fixture.
* Tasks: Added new app argument to GroupResult.restore
  This makes the restore method behave the same way as the GroupResult constructor.
* Tasks: Fixed type checking crash when task takes *args on Python 3
* Documentation and examples improvements