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).

`diff/block_images` was added so the flag and turn images show up in diff scripts.

I noticed that the [blocks in the diff](http://i.gyazo.com/66d38e2fc2ce4d87e6b3f5c708d7d074.png) don't look as good as the [ones in the regular generator](http://i.gyazo.com/07d7547bf6decfc48a1176f794ab85a8.png) so I tried to replace `diff/scratchblocks2.css` and `diff/scratchblocks2.js` but it ended up glitching everything out so I reverted those.