title: cancel dispatch_after() method?
body: <p>Is there a way to cancel <strong>dispatch_after()</strong> scheduled for some time in future, and haven't fired so far? 
  I'm trying to make something like a scheduler for updates from server, and this method is just like I want, but, I'd love to cancel and re-schedule it at some point. 
  Is it possible at all or I have to fallback and use NSTimer?</p>