title: Preload next (currently invisible) cell UICollectionView
body: <p>I'm working with a UICollectionView and currently have full screen cells and when I swipe to the next cell the image takes a bit to load into the cell (since they're high res). I'm already using PHCachingImageManager to cache the image before hand and it's already cached before I load the cell, but just loading the image into the imageView takes a noticeable spark of time. I was wondering if there's a way to preload the invisible next cell BEFORE it gets to cellForItemAtIndexPath?</p>
  
  <p>Any thoughts?</p>