Adding CSS `url()` rewriting

Hi there!

I was having some problems with `url()` statements in minified/bundled CSS files containing relative URLs. So, I added URL rewriting capability to Bundle up. I've played with it a bit and it seems to work just fine. The unit tests seem to be OK too.

I've also added a `generatedDir` config option, just because it seemed right :) I hope it's OK with you.

Could we have this in the official release? I tried to carefully check my code since I'm quite new to CoffeeScript. So, feel free to correct it.

Thanks!