Added optional support for iOS 6's StoreKit

StoreKit's [SKStoreProductViewController](http://developer.apple.com/library/ios/#documentation/StoreKit/Reference/SKITunesProductViewController_Ref/Introduction/Introduction.html) allows the user to rate without leaving the app.

The standard `openUrl` option is still used for pre-iOS 6 versions and cases where a developer doesn't link the StoreKit framework.