Changes to work on iOS 6.
Remove unnecessary tracking of Game Center's availability.
Convert to ARC, use ObjC types.
Thread safety, and consolidation of coding style with the original style.
More style fixing.
More thread safety.
Invitation handling.
Added 2 methods to query number of total and completed achievements.

Added 2 methods:
- (int) numberOfTotalAchievements;
- (int) numberOfCompletedAchievements;

Nothing fancy, very simple, but this is information I needed and I
didn't want to do the maths on every page I wanted to show it.
Merge pull request #7 from Julioacarrettoni/master

Added 2 methods to query number of total and completed achievements.
Fix leaderboard submission.
Add isLocalPlayerAuthenticating property.
Fixes for 64-bit compilation.
Merge remote-tracking branch 'upstream/master' into iOS6