Fix/opera pixel ratio

This is related to [issue #43](https://github.com/yui/yuicompressor/issues/43)

Rules with media queries with a slash in them like `(-o-min-device-pixel-ratio: 5/4)` would break if the block was left empty. 

I replaced the `\` with a string then added it back after the regex that removed empty rules. Ideally something should be done to that regex (on line 350):
```css
css = css.replaceAll("[^\\}\\{/;]+\\{\\}", "");
```