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
eventlet/gevent: Use kombu.async.timer directly
Docs requirements now depends on amqp master.  Closes #2060
Fixes flakes after flake8 update
Programs: celery inspect/control now supports --json argument
Removes debugging print statement
Tests passing
App after forkers were not running due to errors being silenced.  Closes #2055
Add myself as a contributor
Yaml safe_load does not handle exceptions.  Closes #2051
Default serializer is now json
Adds grow/shrink methods to eventlet TaskPool
Tests passing
Merge branch 'master' of github.com:celery/celery