Fixes conversion of paths to web urls for windows

Fixes #3 
Thanks for the fix, I don't have a Windows machine to test, but everything seems ok, with my tests on Linux. One comment before I merge this: I'd change
```javascript
return slash( path.join( '/', path.relative( root, file_path ) ) );
```
with
```javascript
return slash( path.join( path.sep, path.relative( root, file_path ) ) );
```

Also, make sure all tests pass on your Windows machine (`grunt test`). Thanks, sorry for the late review!!