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.
Merge remote-tracking branch 'upstream/master' into iOS6