Issues/149

Previously, every time an image would load in the reader detail, the text would be redrawn.  This is an expensive operation to improve performance I'm changing two things:

First: if the image's frame doesn't change after being loaded skip redrawing text because there is no need. 
Second: Queue up images to load in batches and only redraw the text after a batch of images has been loaded. 

Currently batches are loaded in 10s. 

So, with this change, a post that has 15 images will only redraw text 4 times instead of 17:
once on `viewWillAppear`
once for the timer workaround,
and twice for the two batches. 

The downside is that placeholders are visible longer than they otherwise would be. Maybe it makes sense to do away with the placeholders all together. 

Video items are not loaded via the queue because we fetch those thumbs after a call to the service API.  Large numbers of videos in a post is something that proabably happens rarely so I think its fine to leave them out for now.