Losslessly recompressed images

[Smush.it](http://smush.it/) is a website that will losslessly recompress images. This means that every pixel will be exactly the same (including PNGs with alpha and JPEGs) but the file sizes will be smaller.

If others want to use it, some caveats (that I've handled properly in this pull) when using it:
* Choose to download a ZIP file to get the images without mangled filenames.
* I doubt we'll use GIFs, but it might change nonanimated ones to PNGs.
* It will rename `favicon.ico` to `favicon.ico.png`.

I've heard talk about Gittip. If this is used, I changed a lot of files but didn't do any creative work, so I'm not sure if that's handled properly.