title: Game Center achievements when not signed in to it
body: <p>I am building a game which uses achievements currently.</p>
  
  <p>Let's imagine the following situation:
  The user starts the app the first time, does not sign in to GameCenter and gets a few achievements. But now he decides to use GameCenter and signs in some time later.
  What happens to the gained achievements? Will they be sent to the GameCenter servers later on or do I need to program a check-for-gained-achievements method manually? What did you do?</p>
  
  <p>Thanks for your help!</p>