Minify css with clean-css

The reasons for this change are:

1. better compression
2. clean-css works around a couple of old IE issues like the one which needs a space after comma in `filter` properties (hence bootstrap-theme.min is bigger with clean-css)
3. clean-css is in active development and next thing is to improve selectors merging so compression should improve even more

For bootstrap.min.css

```
102.931 master.css
100.728 clean-css.css
```

/CC @mdo