Add placeholder images

With it, add a category on UIImage that lets us easily create
1x1 px size images of a particular color.
Use NSOperationQueue for AFImageRequestOperation tasks

This lets us (a) control the number of concurrent operations (currently set to 8),
and (b) cancel all operations when the view is about to disappear.
Dequeue cells instead of caching

Also add indexOfObject method to LFSContentCollection, move NSDateFormatter
initialization to static/class methods and remove some dead code.
Correctly import runtime.h
Merge pull request #41 from Livefyre/dev

Generate placeholder images, refine cell reuse pattern