Stops MongoDB Backend from using BROKER_USE_SSL.

This change effectively disables SSL as set using BROKER_USE_SSL.  SSL support
should be functional through existing mechanisms, but requires doc changes and
testing.  I've documented this potential feature and the necessary work with
celery/celery#1974.
Add Brian Bouterse to AUTHORS and CONTRIBUTORS

Conflicts:
	CONTRIBUTORS.txt
Adding changelog entry
Fix for chord inside chord when using json serializer.  Closes #1987
Merge branch 'master' of github.com:celery/celery
Tests passing;