Result cache populated by join_native even if cache disabled
Stresstests: Delete results after bigtasksbigvalue test
Task: Do not send error emails for expected errors (@task(throws=...))
Removes duplicate _set_cache.  Closes #1940
Stresstests: Backend may not implement .forget
Result: .forget() should also clear local cache
Fixes test weirdness
Implements ResultSet.backend (Issue #1936)
Task callbacks applied as group means trail (.children) stored multiple times.  Closes #1936.  Closes #1943
More tests for empty groups
get_backend_cls cannot be memoized as it keeps reference to app.loader.  Issue #1949
Fixes backend tests for Issue #1949
Fixes memory leak when app is registered in multiprocessing after fork registry.  Closes #1949
Update periodic-tasks.rst

Midnight added as an occurance
Add option to worker to control heartbeat interval.

Thanks to my colleague Craig Northway for the patch.
Update task.py
Update tasks.rst
Try to fix CI tests
Make celery.five importable from outside.  Closes #1968
Fixes tests