Move ModelSerializer field_mapping into an attribute for easier overriding.
Drop unused import
Auth is no longer lazy.  Closes #667.

More consistent auth failure behavior.
Release notes
Merge branch 'master' into oauth2-authentication

Conflicts:
	rest_framework/tests/authentication.py
Improve the `django-oauth2-provider` import block

to avoid naming collision with `oauth2` used for OAuth 1
Use the correct doc link style
Clean up some print and comments