Redis-limpyd

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* Add a `queued` field to the `Job` model to enhance avoiding multiple same jobs to be added

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* Add missing scripts module in package

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* Correct bug passing queue_model/error_model/callback to worker script
* Tweak logging

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* A worker can now manage many queues with jobs from many models
* No need to pass `job_model` as it's saved in queues with the jobs pks
* More methods to enhance jobs (`on_*`)
* Add a `max_duration` argument to the worker
* Correct dry-run mode

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* Config for nosetests
* Full test coverage
* Correct a bug with delayed jobs if many workers

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* Jobs can be delayed (when created and/or requeued in case or error)