Update test settings and utils for Django1.7

 - Set package name explicitly on INSTALLED_APPS. Fixes "Item" import
   issue.
 - Add MIDDLEWARE_CLASSES to suppress warning.
 - Add "enter" keyword to util sending ``setting_changed`` signal:
   https://docs.djangoproject.com/en/dev/releases/1.7/#signals
 - Change ``mimetype`` to ``content_type``
   https://docs.djangoproject.com/en/dev/releases/1.7/#features-removed-in-1-7

After this changeset, all tests pass with the exception of exif orientation checks which seem to differ between some backends.