Fix paths in generated metadata.json on Windows

When using gulp-modernizr, feature detects in subdirectories throw an error on my Windows 7 VM. I've narrowed it down to ```lib/generate-meta.json``` creating weird paths:

![image](https://cloud.githubusercontent.com/assets/654171/3363220/b68e2788-fb11-11e3-82a3-d271bf8ac08e.png)

Example: ```./feature-detects\css/all.js```

/cc @patrickkettner @doctyper