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
Update periodic-tasks.rst

Midnight added as an occurance
Merge pull request #1959 from AbsoluteMSTR/patch-1

Update periodic-tasks.rst
Docs: generic celerybeat options should not write schedule to root owned directory.  Thanks to Nikos Fertakis.
Merge branch 'master' of github.com:celery/celery
Add option to worker to control heartbeat interval.

Thanks to my colleague Craig Northway for the patch.
Update task.py
Update tasks.rst
Merge pull request #1970 from dtheodor/master

Fix documentation for Task.throws
Merge branch 'master' of github.com:celery/celery
Merge pull request #1966 from mduggan/worker-heartbeat-option

Add option to worker to control heartbeat interval.