Game Center/Guide overhaul for iOS 6+

Partially overhauled the Guide for iOS 6+ Game Center functionality:
Encapsulated repetitious code into single-purpose methods, fixed runtime
warnings for Game Center in iOS 7, and iOS 6+ no longer uses deprecated
methods to open Game Center windows (all compiler warnings in Guide.cs
are fixed). Also fixed a MatchMaker crash bug. Also fixed the formatting
(tabs, spaces, braces) in Guide.cs to be in line with MonoGame's coding
conventions. Bonus feature: Added access to iOS's Twitter API to the
Guide now too. Tested on iOS 5, 6, and 7.