Implement the ability to run all available jobs and exit

This implements 2 new commands that allow you to run all available jobs and the exit when there is nothing left to do.

rake jobs:workoff
Is identical to rake jobs:work but will exit when no more jobs are available to run

Also --exit-on-complete has been added as a command line flag to ./script/delayed_job which will cause the process to exit after there are no longer available jobs to run.