Venmo library updates for iOS 7

Apple no longer accepts submissions for app that use [[UIDevice currentDevice] uniqueIdentifier]. I've updated the code to use the new identifierForVendor method in iOS 6 & 7 and added code for an alternative method for iOS 5 and down. I've tested this with the Splitwise app and this looks like it works correctly.

I've added files from https://github.com/gekitz/UIDevice-with-UniqueIdentifier-for-iOS-5 for the iOS5 legacy identifier.

Marshall