Css concatenation breaks paths if concatenated css is at a different path to source css.

If you have a folder like: 

```
/css
  one.css
  two/
    two.css
    img.png
```

and in two.css you have: 

```
.style {
  background-image: url('img.png')
}
```

After concatenation the css will break because it's trying to load 'img.png' from a file called: `css/css-1111.css`.

That is - the paths aren't maintained.
