Reworked SignedInGamer for full iOS 4-8 compatibility

The `SignedInGamer` class was recently updated to remove the old deprecated Game Center methods for authentication, leaderboards, and achievements. That's a great thing for iOS 6+, but in the process we unfortunately lost support for iOS 4.2 through iOS 5.1.

Since we had code that works perfectly for iOS 4.2-5.1 (and my game still supports iOS 5), I figure why not just support both the old way and the new way? This PR adds a version check and calls the right code depending on iOS version so that we use the new methods where possible, or fallback to the old if need be.

**Note:** The diff shows way more additions/deletions than I actually made, since the file was autoformatted to conform to the MonoGame coding guidelines. Most of the changes are just whitespace differences.