Upgrade to Django 1.7.1

This commit upgrades the project to Django 1.7; the changes here are:

 - admin.autodiscover() is no longer required

 - This switches from South to the new core migrations (among other
   things, this means that syncdb is no longer required as a separate
   step)

 - Upgrade nose and django-nose to avoid a deprecation warning about
   doctest versions:

      RemovedInDjango18Warning: The django.test._doctest module is deprecated;
      use the doctest module from the Python standard library instead.