Please don't automatically merge this!

I've made some modifications to the SSKeychainQuery class by adding some additional properties and methods that essentially allow you to use Touch ID with SSKeychain.

The main change is that i've modified the save: method to support the kSecAttrAccessControl and kSecUseNoAuthenticationUI attributes allowing you to specify a SSKeychainAccessControl object via the new accessControl property (was wrapping this in an NSObject the best approach?).

I have also made changes to fetch: and fetchAll: allowing you to pass in the kSecUseOperationPrompt (displayed when Touch ID is presented) via useOperationPrompt.

Finally, I've added a update: method as there was no existing wrapper around SecItemUpdate().

With all the above changes, You can now do something like the following to protect items with Touch ID:

SSKeychainQuery *query = [SSKeychainQuery new];
query.service = @"SomeService";
query.account = @"SomeAccount";
query.password = @"SomePassword";
query.accessControl = [SSKeychainAccessControl accessControlWithAccessibility:SSKeychainAccessibilityWhenPasscodeSetThisDeviceOnly flags:SSKeychainCreateFlagUserPresence];

NSError *saveError = nil;
if (![query save:&saveError]) {
    NSLog(@"Failed to save item to the keychain: %@", saveError);
} else {
    NSLog(@"saved item successfully");
}

Then, you can fetch them like so:

SSKeychainQuery *query = [SSKeychainQuery new];
query.service = @"SomeService";
query.account = @"SomeAccount";
query.useOperationPrompt = @"A prompt displayed when Touch ID alert is shown";

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSError *fetchError = nil;
    if (![query fetch:&fetchError]) {
        NSLog(@"Failed to fetch item from the keychain: %@", fetchError);
        // `errSecAuthFailed` indicates that the user cancelled the prompt
    } else {
        NSLog(@"got password: %@", query.password);
    }
});

I've not really done much when it comes to maintaining backwards compatibility but I've tested my branch when compiled against the iOS 7 SDK and it still compiles but I would like it if somebody else could check over everything beforehand.

Apple Sample Code: https://developer.apple.com/library/prerelease/ios/samplecode/KeychainTouchID/Introduction/Intro.html

Let me know what you think.

Liam