title: General design - Where do I put centrally accessed objects
body: <p>I have my main app delegate
  I have a few UIViewController derived instances driven by a Storyboard</p>
  
  <p>Say I'd like to provide a centralized persistence layer for my application - perhaps Core Data of SQLite. Where would I put those objects? I'm missing some centrally accessible "Application" class you can access from all the UIViewController instances. </p>
  
  <p>Is there a pattern to follow here?</p>