job.cancel() now deletes dependents_key
Removed unnecessary lines from test_job.py.
Merge pull request #266 from selwin/cancel-dependents

Canceling a job should also delete its dependents_key
First stab at allowing user to specify a custom worker class.
Renamed "--worker" flag to "--worker-class".
Let's keep things readable.
Merge branch 'selwin-custom-worker'
Fix typo in Doc string.
Merge pull request #268 from reterVision/master

Fix typo in Doc string.