Add image compression to lib

```
I'd love to see image compression added to your already excellent lib. You
already compress html, css, and js, so why not images as well? Images are some
of the biggest resource hogs on a website, so they're a good candidate for
inclusion. I'm thinking it could be as simple as adding support for tools like
pngcrush (or any other PNG optimizer tool) and/or jpegtran for JPEGs. You
already include external dependencies for yuicompressor, so adding these other
tools shouldn't be that much of a stretch. Or, am I wrong? I've been looking
for an all around solution for quite some time now... and you're lib is the
only one that even comes close (besides google's mod_pagespeed which I can't
use).
```

Original issue reported on code.google.com by `ram...@gmail.com` on 28 Mar 2011 at 8:34