title: something like cron (timer) in gevent
body: <p>How can I make classical timer in gevent?</p>
  
  <p>I'm currently using gevent.spawn_later but is there a way to use core.timer like saying "do callback on interval" just like cron does?</p>
  
  <pre><code>def callback():
      # do something
      print '!'
  
  timer = core.timer(10, callback)
  timer.start()
  
  outpout:
  0s
  10s  !
  20s  !
  30s  !
  </code></pre>