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
fix typo
Wording
Use numbers module
Merge pull request #1868 from outself/patch-1

fix typo
Redis backend chord optimization: Avoid save group at apply and pipeline + O(1) join.

This change is backward incompatible and so is not enabled by default.

To enable this optimization you have to set the `new_join` option
and it must be enabled by all clients and workers part of the chord::

    redis://?new_join=1
Merge branch 'master' of github.com:celery/celery
Added a small cache template for stress test so I don't piss of the OOM killer :)
Add an execv template for stress tests.
Tests should not create files