title: How to save and retry reporting GKAchievement's after network failure?
body: <p>Apple states that if you want to report a GKAchievement but you get a network error, the best way to handle this is to save the GKAchievement (possibly adding it to an array), then periodically attempt to report the achievement.</p>
  
  <p>What is the best place to save the achievements? Would NSUserDefaults suffice, or would a property list be a better way?</p>
  
  <p>When and how often should I attempt to report? On application launch, or something like every 10 minutes?</p>