Added MGImageUtilities pod (custom version)

Our custom version allows images without alpha information
Fixed huge memory leak stripping HTML
Free up memory after updating a reader post
Added featuredImageURL to ReaderPost

We'll use that to check for existence of a featured image, since an image with a URL we can't fetch is as good as a post with no image
Added WPTableImageSource class
Use WPTableImageSource to fetch featured images

Some other performance improvements:

* Use integer metrics for everything, UIKit doesn't like painting stuff at fractional pixels
* Removed cell shadow until we find a way to make it scroll smoothly
* No more transparent backgrounds
* Cache placeholder blavatars
Merge branch 'reader-imagesource' into reader