Added platform specific file separator

Hi,

we encountered a problem with the navigation dropdown menus. Say we have a directory structure like the following

```bash
styles
 --- editor
 --- --- one.css
 --- --- two.css
 --- default
 --- --- three.css
 --- --- four.css
```

then on `win32` we get
![screen shot 2014-03-24 at 10 20 32](https://f.cloud.github.com/assets/3540036/2497421/cf632c2c-b336-11e3-85a2-ca1eb1eebdd2.png)

and on `unix` we have
![screen shot 2014-03-24 at 10 12 12](https://f.cloud.github.com/assets/3540036/2497434/e5b9bfae-b336-11e3-9b95-7413e810a768.png)

With this commit the result is always the one from `unix`.