Add CsrfViewMiddleware to MIDDLEWARE_CLASSES

This middleware is enabled by default in Django, but since we are
overriding MIDDLEWARE_CLASSES, we need to add it here.
https://docs.djangoproject.com/en/dev/ref/csrf/#how-to-use-it
Merge pull request #33 from ccnmtl/add-csrf-middleware

Add CsrfViewMiddleware to MIDDLEWARE_CLASSES