* support polymorphic list

Render polymorphic list of items with different fields

* pep8

* fix polymorphic models serializer

fix get_polymorphic_serializer_for_instance only for PolymorphicModelSerializer

* fix typo

* pep8

* fix circular import and pep8

* Fix import in renderers

* Fix remaining flake8 issue

* Fix code style issue in renderers.

* Add Adam Ziolkowski and Roberto Barreda as Authors

* Add DRF 3.7 to the test matrix.

Fixes django-json-api#388

* Switch to an explicit build matrix.

* Change env format.

Without the equals sign, Travis will drop the double quotes and screw
up the environment variable export.

* DRF 3.6.x does not support Django 2.0.

* Add required on_delete to ForeignKey fields.

* Add on_delete to OneToOneField.

* Switch core.urlresolvers to urls for Django 2.0 compatibility.

* Update minimum required django-polymorphic.

Older versions of django-polymorphic don't support Django 2.0

* Remove factory-boy version constraint.

* Remove constraint on pytest version.

It appears that pytest-factoryboy now works with a new version of
pytest.

* Missed a place where pytest version was constrained.

* Force use of a newer pytest.

* Make documented example runserver work

Resolves django-json-api#410: docs/getting-started.md example doesn't work.

* xGet the steps right in the documentation to run the example app.

* First attempt at for issue django-json-api#410 didn't have includes sorted properly.

- travis caught it. This time I've done the isort locally to confirm the order.
- MIDDLEWARE_CLASSES was replaced by MIDDLEWARE as of Django 1.10 and, according to
  tox.ini, this is only testing >=1.11.

* missed a line which flake8 caught

* add back install of this package

* add myself as an author as requested

This branch adds a skeleton of contribution guidelines and cleans up
little bits that I noticed along the way.

For django-json-api#424

The metadata generation was using the model name as the resource name
even when a field had an included serializer with a different
resource name.

 * Cache included serializers instead of looking up every time.
 * Cache inflections.
 * Skip some validation, optionally.
 * Try and use `queryset` before `get_queryset`.

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