title: Create an NSString with leading zeroes for minutes and seconds
body: <p>I need to constantly update the a playback time in a label. I am getting the time <code>NSTimeInterval currentProgress = self.moviePlayerController.currentPlaybackTime;</code> which returns a double value. But the format in which I need to display is "MM:SS" (00:00). I have tried the following code </p>
  
  <pre><code>NSTimeInterval currentProgress = self.moviePlayerController.currentPlaybackTime;
  
  float min = floor(currentProgress/60);
  float sec = round(currentProgress - min * 60);
  </code></pre>
  
  <p>When the progress time is 10 minutes and 10 seconds it displays perfectly fine, as "10:10". But the problem here is if the progress time is 1 minute and 7 seconds, it displays "1:7" in the label. My intended  display is "01:07". How can I do that?</p>