title: simulate crontab with twisted deferred and looping calls
body: <p>I would like to implement a cron-like behaviour with my twisted application.
  I want to trigger a periodic call (let's say every week) but running at a precise time only, not when i start my application. </p>
  
  <p>My use case is the following:
  my python application is started at any time in the week.  I want the calls to be performed every monday at 8am.
  But I don't want to perorm active waiting (using a time.sleep()), i would like to use callLater to trigger the call next monday and then start a looping call from that date.</p>
  
  <p>any idea/advice?
  thanks,
  J.</p>