Merge pull request #54 from brocaar/worker_pool_lb

Optimize authorization query.
Move reschedule and send_error_notification calls to signal instead of RunResource class.
Re-orden methods.
Move reschedule failed notification to notifications module.
Move run failed notification to notifications module.
Add run_on_all_workers field to Job model.
Change schedule_now to schedule.
Make reschedule use the schedule method when dts has been calculated.