3.8 compat

I made some updates to make the UI work better in 3.8. Without these edits, the modal looks broken. Props to Herbert on the WordPress.org forums for figuring out that fix: http://wordpress.org/support/topic/searchbox-missing

I also made the thumbnails retina compatible.

Additionally, I changed the big question mark image to "Set Featured Image" so the UI matches the WP post featured image UI. I also removed the +/- buttons and changed that to just "Remove Featured Image"

I also fixed two bugs where if you associated an image and closed the modal, then opened the modal again, it wouldn't show the "Remove association" link for that image. Similarly if you removed an association in the modal, and then closed and reopened the modal, it wouldn't show the "Associate.." button.

If you want me to break this up into smaller pull requests, let me know. :)