Serves static files with optional directory index
Files in static folder are automatically served on static URL by Flask. See http://flask.pocoo.org/docs/0.10/api/#application-object.
It’s recommended to specify static folder and URL path directly on Flask application object, unless you need additional static folders, or have multiple route handlers for the URL path, e.g. when serving static files on root URL (‘’) for any path unmatched with previous routes.