Import unittest2 from Django

This commit solves two problems:
 - Django 1.7 uses the stock python unittest module instead of using the unittest2 backport. This is an issue as when we raise the Skip and ExpectedFailure exceptions, these are seen by Django as errors. This commit makes sure that we always use the same unittest module that Django is using to prevent this from happening.
 - Removes the need to install unittest2 (by importing it from django.utils.unittest instead)