func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {

    var slider = PhotoSlider.ViewController(imageURLs: self.images)
    slider.modalPresentationStyle = .OverCurrentContext
    slider.modalTransitionStyle = UIModalTransitionStyle.CrossDissolve
    slider.index = indexPath.row
    self.presentViewController(slider, animated: true, completion: nil)

}