ARC, iPhone 5, iOS 6 compatibility; improved error reporting / handling; GC Challenges; improved demo app

This pull request makes major improvements to almost every class and file in GameCenter Manager. The biggest improvement is ARC support. All files (including those from third-parties) now use ARC. Many old methods have been updated that were depreciated with the introduction of iOS 6.

We have also improved error reporting by adding a `kGameCenterManagerErrorNotification` that will notify its classes when there is an error with a process. The `isGameCenterAvailable` is now set to `NO` when there is an error with GameCenter (ex. player / game authentication or connection) *and* when older devices attempt to use GameCenter. 

Added a new method to retrieve a list of GameCenter Challenges for the current game and player.

The demo app has undergone massive improvements, including many interface improvements (converted to storyboards), iPhone 5 support, a new icon, and better GC status reporting.  Reorganized files to remove duplicates. Demo app only supports iOS 5+, but the GC Manager still supports iOS 4+