provide ability to jobs to add themselves back to the queue

I found this module useful, but I was vexed as to how to allow my jobs to re-add themselves to the queue without having the job queue cron run for the full 1/2 length of the maximum php execution time (as per job_queue_cron). I added a counter that will limit the job_queue to either half the maximum execution time for php or the total number of jobs (whichever comes first).