Support for running the test suite with py.test

 * Get rid of runtests.py
 * Moved test code  from rest_framework/tests and rest_framework/runtests to tests
 * Invoke py.test from setup.py
 * Invoke py.test from Travis
 * Invoke py.test from tox
 * Changed setUpClass to be just plain setUp in test_permissions.py
 * Updated contribution guideline to show how to invoke py.test
Merge remote-tracking branch 'pelme/pytest' into feature/pytest

Conflicts:
	.travis.yml
	rest_framework/runtests/runtests.py
	tests/test_filters.py
	tests/test_pagination.py
	tox.ini
Upgraded to pytest-django 2.6.1
Fixed the issue with django-filters / django 1.7 / pytest
Merge remote-tracking branch 'reference/master' into feature/pytest

Conflicts:
	tests/test_serializer.py
Merge remote-tracking branch 'reference/2.4.0' into feature/pytest

Conflicts:
	rest_framework/runtests/urls.py
	tests/test_response.py
	tox.ini
Added missing staticfiles app
Use urls functions from django instead of compat.
Updated the release-notes.