Fixes jsTree's relative URL construction for theme components

I had some styling issues appear in the admin interface of django-cms after deploying my static files to a location other than '/static/...' (Screenshot: http://jonprindiville.net/django-cms-admin-styling.png )

Turns out that jsTree tries loading some theme components from paths relative to the URL in the browser's location bar. When initializing jsTree, here, I've written a function to find the path of the URL used to load this script and then set jsTree's theme_path appropriately.

(This file was already full of tabs, not spaces, so I have left them as-is)