Allow to specify any custom arguments for celery workers. This will allow to insert "-Ofair" in celery 3.1 wihout making the options bloated on  sparks side.
Don't make my defaults be the defaults for everyone. Spotted by @Eliotberriot.
Missing space, very important for arguments chaining.
Empty the README and refresh the remaining parts. Contents is in the Github Wiki now. More easily editable by contributors (and me, too).
version bump
Merge branch 'release/3.12'