Release notes for 3.1.1 and version bump.

Use `serializer_class` for browsable API display, even on plain APIView.

AuthTokenSerializer - properly render password field

Put the needed import in one example

Fix typo in versioning docs

use SAFE_METHODS and make tuple

Add necessary imports to final urls.py example

Updates ducumentation DEFAULT_PAGE_SIZE left over after #2655

Do not paginate if PAGE_SIZE=None

Allow blank/null on radio.html choices

Simplify test settings

fixed missing anchor closing tag

when next_url is none, big part of page html will be rendered under the <a href='#'> as it does not have a closing tag.

Fix lookup_url_kwarg handling in viewsets (Fixes #2591).

The ``lookup_url_kwarg`` is intended to set the name of a field in the
URL regexps when using custom ``lookup_field``, but the routers ignore
it altogether.

Fixed problem with imporing rest_framework.views in apps.py

Fix example of explicit format URL (for real).

Do what I meant to do in #2670:

The link is displayed in the tutorial as "http://example.com/api/items/4.json" but the hyperlink itself and what the example code actually works with are (in my opinion, less pretty) URLs like "http://example.com/api/items/4/.json".

Fix for minor documentation typo

Merge in from upstream

Changed incorrect class name in pagination docs