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
Implements steeves dynamic tasks as task.replace. Issue #817
Use a heapq to avoid calling all entries for every iteration
Remove print
Modify some examples
Wording
Added a small cache template for stress test so I don't piss of the OOM killer :)
Add an execv template for stress tests.
setting for optionally syncing on every scheduled task execution
worker.heartbeat: Update 'interval' doc default to 2 seconds

Since the 30-second docs were written in 3eaffda (Hartbeat is now 30
seconds, but documented as 2 minutes, 2011-06-13), the default has
changed twice:

* It was reduced to 5 seconds by 387cf1c (Heartbeat frequency now
  every 5s, and frequency sent with event, 2012-03-28).
* It was reduced to 2 seconds by 6008303 (Persist clock value and
  synchronize revoked tasks with others, 2012-11-05).
Merge pull request #1879 from wking/worker-heartbeat-docs

worker.heartbeat: Update 'interval' doc default to 2 seconds
Adds amqp to list of packages
documentation, finer grained control on syncing, tests
Fixes typo