Added support for iOS 8/OS X 10.0 AccessControl (Touch ID)

**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