Support basic authentication with custom user models that change username field

On Tom's suggestion, improve coding style by using a single-line call to getattr() with a default value instead of a multi-line if/else clause.

Fix flake8 error

Support User model in Django 1.4 that has not a USERNAME_FIELD attribute.

Import get_user_model from compat module to be compatible with older django versions (e.g. 1.4).

Compatibility code for getting user model

Support basic authentication with custom user models with a username field that is not named 'username'.

Allow unexpected values for ChoiceField/MultipleChoiceField representations

Release 3.1.2

Allow DjangoObjectPermissions to use views that define get_queryset

minor grammar fix

Adding django-versatileimagefield to DRF 3rd party docs

Closes WGBH/django-versatileimagefield#12

Added link to djangorestframework-word-filter package

update info about wq.db.rest router

Separate LICENSE and README to two files.

Document correct method usage for get_paginated_response

Delete imported but not used method

Delete imported but not used method

 In 'Generating Tokens' section, the example code imported get_user_model but not used.

Fixed description for get_paginated_response method in generic-views and example using it with @list_route decorator in viewsets.

Fix Invalid keyword model with generic views

DateField to_representation can handle str and empty values. Fixes #2656, #2687.

of old get_pagination_serializer only valid in 2.x

Depth should reduce on nested ModelSerializer classes.

Merge pull request #2757 from ekiourk/2630/Raise-exception-when-ModelSerializer-used-with-abstract-model

Raise error when ModelSerializer used with abstract model

Update 5-relationships-and-hyperlinked-apis.md