Added integration_tests app to example project

- added integration_tests to tox
Cache content types for classes.
Change get_objects_for_user so it creates much more efficient queries when not using groups.
Change get_objects_for_user so it creates much more efficient queries when not using groups.
Whoops.
Add test to give get_objects_for_user 100% coverage again.
Merge faster_get_objects_for_user. Fix conflict.
Support usage without anonymous users

With some custom models, creating users may be complicated; reasonable
defaults may not exist for some fields, and special-casing a sentinel user
elsewhere in views code separately from Django's provided AnonymousUser may
be unpalatable.

For these situations this commit adds the ability to disable anonymous user
support entirely; any permissions check against such a user will always
return False. No user is created when "syncdb" is run.

To disable anonymous user support in your site, set ANONYMOUS_USER_ID to
"None" in settings.py.

An unset ANONYMOUS_USER_ID will still cause an error.
Document ANONYMOUS_USER_ID=None in configuration.rst
GuardedModelAdmin patch for supporting USERNAME_FIELD
error message fixed, tests run when "testsettings.py" is used
Imported User and Group models in the assign object permissions tutorial
Merge branch 'master' of https://github.com/lukaszb/django-guardian
add accept_global_perms into PermissionRequiredMixin
Merge branch 'master' of github.com:mojeto/django-guardian into mojeto-master
Merge branch 'docs' of github.com:migueldvb/django-guardian into migueldvb-docs
Added Miguel to the AUTHORS file
repair description in PermissionRequiredMixin
Merge branch 'master' of github.com:mojeto/django-guardian into mojeto-master
Merged from mojeto master branch