Simple priority submission feature for WQ.
Modified send_one_task to iterate over the ready list, so as to keep tasks in priority order.
Factored push_task_to_ready_list so that all inserts follow the same logic.
Remove unused stoptimes.
Added priority to the python bindings.
Fix binding documentation.
Adds priority test.
Deprecates specify_task_order.

We could not gurantee the given policy. With the addition or
specify_priority, the queue works as if FIFO was specified.
Merge pull request #558 from btovar/work-queue-priority

Work queue priority