Adding a check for anonymous user and getting its objects. Added tests for covering the new use case.

The bug fixed is that if we’re passed an django.contrib.auth.models.AnonymousUser the database queries will fail. Since this user is often used in requests it makes sense that we should use django-guardian’s own AnonymousUser instead.
Merge pull request #187 from hansl/devel

Adding a check for anonymous user and getting its objects.