Fix url template tags for Django 1.5

In Django 1.5 url tags will spit out NoReverseMatch errors such as:

NoReverseMatch at /mce_filebrowser/image/
Reverse for ''mce-filebrowser-remove-image'' with arguments '(1L,)' and keyword arguments '{}' not found.

This patch add {% load url from future %} in the templates.

Tested on Django 1.4.5 and 1.5.1