Dealing with App Store subscriptions

Added $storekit.loadReceiptForProduct(), $storekit.loadAppStoreReceipt() and $storekit.validateReceipt() for dealing with App Store subscriptions.

Example usage:

    //iOS >= 7
    $storekit.loadAppStoreReceipt().then(function(appStoreReceipt){
        $storekit.validateReceipt(appStoreReceipt).then(function(response){
            console.log(response);
        });
    });