allow previously loaded larger images to be used at lower media sizes

Marks sources that have already been loaded using a data attribute, then includes these "cached" sources in the list of matches. This prevents the script from loading a new image if a larger one has already been used.

Saw this comment and figured I'd take a crack at it: http://css-tricks.com/hassle-free-responsive-images-for-wordpress/#li-comment-1513380