Added the mixins LoginRequiredMixin, PermissionRequiredMixin and
UserPassesTestMixin to contrib.auth as counterparts to the respective
view decorators.
The authentication mixins UserPassesTestMixin, LoginRequiredMixin and
PermissionRequiredMixin have been inspired by django-braces
<https://github.com/brack3t/django-braces/>
Thanks Raphael Michel for the initial patch, tests and docs on the PR
and Ana Balica, Kenneth Love, Marc Tamlyn, and Tim Graham for the
review.
The code examples should now work correctly.
The `get_context_data` method in the examples was changed when
necessary to adopt a singular style (get context with super(...),
add the extra keys to the dict then return it).
Thanks to Remco Wendt for the initial report and to Tim Graham
for the review.
