Ensure that subtask argument updating in chain and group respects subtask immutability
Removed safe parameter due to PyMongo deprecation warning
Fix for Py3.3 __qualname__.  Closes #1356
Fix for celery.utils.threads.Event. Re #1354
Fix for broken build 721 due to safe parameter
Fix for broken build 721 due to safe parameter
Update CONTRIBUTORS.txt
Merge branch '3.0'

Conflicts:
	celery/utils/imports.py
	celery/utils/threads.py
worker_shutdown signal no longer sent.  Closes #1339
Task.apply/always_eager now executes link/link_error.  Closes #1336
Fixes typo with TIME_LIMIT default for tasks
Merge branch '3.0'

Conflicts:
	celery/worker/__init__.py
Tests passing
Tests for new Result.__iter__