Preserve default ordering when using filtering

Fix returning None when allow_none is True in CharField

Display validation errors on login form

Hide login link in browsable API if the login view is not registered.

Using user.get_username() instead of user.username.

This solves an error when using a auth model that does not have a username field.

Alter CSRF exemption implementation

Test with released Django 1.7

The previous implementation of decorating `APIView.dispach` with the
`csrf_exempt` decorator allowed for an easy-to-make mistake where
someone could override the `dispatch` method on a view and inadvertantly
remove the csrf exemption of their api view.

By moving the decoration of the view into the `as_view` logic, it
becomes much more difficult to make this mistake.

Use explicit `many=True` in PaginationSerializer on `object_serializer` instantiation

Use explicit many=True for object_serializer instantiation in PaginationSerializer and add catch dummy 'many' kwarg on DefaultObjectSerializer

as per https://github.com/tomchristie/django-rest-framework/pull/1820#discussion_r16987993

Fixes #1738

Correct testing documentation details.