Adding ARSwipeToSelectGestureRecognizer Podspec

[ARSwipeToSelectGestureRecognizer](https://github.com/ayn/ARSwipeToSelectGestureRecognizer) is a custom gesture recognizer that allow selecting multiple cells by swiping. After this Spec is merged into the master repo, I'll submit [ARSwipeToSelectPickerController](https://github.com/ayn/ARSwipeToSelectPickerController), which is a UIImagePickerController replacement that uses this gesture recognizer.