Fixed assets url; added diff/block_images

The Scratch assets URL has been changed from `cdn.scratch.mit.edu` to `assets.scratch.mit.edu` so [costumes now show up](http://i.gyazo.com/66376266b3561f770d4d2904d41aaf52.png).

Also, `diff/block_images` was added so the flag and turn images show up in diff scripts.