Modernise and tidy

I needed to use this in a new project which uses Objective-C ARC and has to run on 64-bit iOS devices such as the iPad Air, iPhone 5s and Retina iPad Mini.

b0c133a makes TDOAuth safe in 64- and 32-bit programs.
2374821 makes TDOAuth build with ARC.
9affaa9 updates TDOAuth to modern objective-C syntax via the Xcode auto-updater
e30df4b cleans up the .h to expose the minimum interface and renames the category methods to reduce the chance of collision with other categories on NSString or NSNumber
a9f94c0 fixes a bug introduced in 9affaa9 which caused a crash when no access_token was available

I have tested this on iOS 7