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
add a groupmeta_collection option to save group results in a different collection than results and add a 'bson' fake kombu encoder to allow pymongo to serialize natively data in mongodb
kombu.utils.get_errno is now removed as it is not needed on Py2.7
Merge branch 'master' into bigbeat

Conflicts:
	celery/beat.py
Fixes typo Publisher -> Producer
Beat: Scheduler.Publisher renamed to .Producer, and .publisher to .producer
Programs: multi must passhthrough %i and %I logfile formats
Default log format is now includes %I to avoid race conditions (one logfile per child process)
flakes
[Taskv2] callbacks, errbacks, chord and chain moved to message body to avoid header limitations.
Task protocol v2 document moved into single protocol document