Merge branch 'master' into rumineykova/master

Conflicts:
	celery/app/defaults.py
	celery/apps/worker.py
	celery/worker/__init__.py
	celery/worker/consumer.py
Fix flakes from master
Refactor Agent
Fix flakes from master
flakes
Moves Agent to cell
Disable agent for now
Tests passing
Merge branch 'cell'
Consistently use 'bootsteps', singleword no spaces or hyphens
Use command-line instead of 'command line'
Starting the new Extensions userguide
Better graph output
Update result graph
Adds worker bootstep graphs to docs
Adds missing images
New 'celery graph' command replaces worker_graph|consumer_graph.  E.g.: celery graph bootsteps worker consumer,  connects both or you can use only one
New 'celery graph workers' command creates graph of the current cluster
New C_IMPDEBUG envvar traces imports, from Armin's talk
Nicer layout for 'celery graph workers'