Correct dry-run mode
Dryification of the list of worker's arguments

(and also enhance the print-options script argument)
A worker can now manage many queues

But all queues must be on the same models, with all jobs on the same
model too.
Move import_class in utils
Store jobs models representation in the queues with their PKs

It avoids passing job_model to the worker, and allow storing many sorts
of jobs in a queue.