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
Fixes test weirdness
Implements ResultSet.backend (Issue #1936)
Task callbacks applied as group means trail (.children) stored multiple times.  Closes #1936.  Closes #1943
Multi: %n is now the same as %N to be consistent with "celery worker".

Closes #1938
More tests for empty groups
Multi doc update
get_backend_cls cannot be memoized as it keeps reference to app.loader.  Issue #1949
Fixes multi tests
Fixes backend tests for Issue #1949
Fixes memory leak when app is registered in multiprocessing after fork registry.  Closes #1949