Support for CSS @import #48

A feature request: Currently mod_pagespeed rewrites css that is pulled in with 
the <link> tag just fine, e.g.:

<link rel="stylesheet" type="text/css" 
href="http://www.mysite.com/mystylesheet.css" />

However, it could be nice it it would work with @import as well, e.g. 

<style type="text/css" media="screen">@import 
url(http://www.mysite.com/mystylesheet.css) ;</style>

We have a lot of sites that are using that pattern.

mod_pagespeed does not rewrite that in my end. If the current mod_pagespeed 
version is supposed to pick up @import I can provide some debug info.

Pagespeed version: 0.9.0.0-128