Improved iOS Game Center functions

Created by: romulox-x

Fixed null pointer bug in ios store. added functions to game center to reset the user's achievements, request all the achievements they've made progress on, request descriptions of all possible achievements, and show the built in game center overlay. Upgraded all (I think) deprecated functions to ios 6 versions. as per discussion with reduz on IRC, ios 6 is now the official lowest supported iOS version. compatibility could be added, but it's so obsolete, it's tough to argue for continuing to support it. some naming of functions and dictionary parameters might need to change. I just named them whatever made sense to me :)