Initial riak support
Add some documentation on class attributes

Also add handling of protocol.
Cleanup code related to connection to Riak

This is a leftover of previous work on this backend.
Switch default bucket name to celery
Add riak to extra requirements
Add Riak backend documentation
Fixed error in protocol handling for the riak backend
Use a heapq to avoid calling all entries for every iteration
Remove print
Stresstests: Backend may not implement .forget
Master branch is now 3.2a1
Stresstests: Refactors Suite into BaseSuite and Suite
TaskProducer replaced by create_task_message and send_task_message
3.2: Use dict and set comprehensions (+ literals)
Removes Py2.6 workaround for missing WeakSet
[3.2] Requires Py2.7+
create_task_message for task protocol v2
Fixes amqp publish
Fixes amqp publish #2
Reorganizes AMQP class