Added drf-ujson-renderer to renderers docs

drf-ujson-renderer is a third party renderer that implements JSON renderering using UltraJSON

Typo on api-guide/fields.md and serializers.py

Fix object creation with reverse M2M when related_name unspecified

Updated versions in tox and travis config

Fixed a regression with ValidationError under Django 1.6

It seems that field.related_query_name() does not return the related_name
for reverse M2M relations when related_name is not explicitly set in the M2M field
definition.

So, change to use obj.get_accessor_name(), where obj is an instance of
RelatedObject, as are returned by a model's
_meta.get_all_related_many_to_many_objects(), or as in the tuples returned by
_meta.get_all_m2m_objects_with_model().

Improve handling of 'empty' values for ChoiceField

The empty value defaults back to '' (for backwards-compatibility) but
is changed automatically to None for ModelSerializers if the `null`
property is set on the db field.

Updated OAuth2 authentication docs.

Fixed exception handling with YAML and XML parsers.

Great. Thanks!

Docs: Fix for API test client example

need to define 'expected' as we can just test against the dict.

Removed commented-out credits from footer so that django-debug-toolbar works

The comment, although valid, caused that the Django debug toolbar's injected HTML was partially commented-out and thus the toolbar didn't work as expected.

Added get_filter_backends method

Typo on generic-views.md

Fixed UnicodeEncodeError when POST JSON via web interface; added test