Use functools.update_wrapper for Django 1.6+ / Python 2.5+


This fixes https://github.com/dcramer/nexus/issues/31

update_wrapper is no longer in django.utils.functional as of Django 1.6, since it has been part of the Python standard library's functools package since Python 2.5.  The import's been changed (and the usage has also been changed to use the package name for consistent coding style -- it seems that stdlib packages aren't imported with "from" in this project, or at least in this file).