url_get: handle filenames on windows

[mistoll]

overlay_url_for uses os.path.sep to split the filename into a list.

os.path.sep is platform dependant, hence the template is platform dependant, which is not desired.

On linux:
url_for('bower.static', filename='jquery/dist/jquery.js')
On Windows:
url_for('bower.static', filename='jquery\\dist\\jquery.js')