title: Get notification when NSOperationQueue finishes all tasks
body: <p><code>NSOperationQueue</code> has <code>waitUntilAllOperationsAreFinished</code>, but I don't want to wait synchronously for it. I just want to hide progress indicator in UI when queue finishes.</p>
  
  <p>What's the best way to accomplish this?</p>
  
  <p>I can't send notifications from my <code>NSOperation</code>s, because I don't know which one is going to be last, and <code>[queue operations]</code> might not be empty yet (or worse - repopulated) when notification is received.</p>