Coerce None to '' in CharField.to_native()

Conflicts:
	.travis.yml
	docs/api-guide/routers.md
	rest_framework/compat.py
	tox.ini

Simplified some examples in tutorial

This reverts commit d6806340.

When many tab's are open (which is necessary for DRF's documentation), it becomes impossible to determine which tab contains which pieces of documentation.  That they are all related is obvious by the use of a common icon, just not the specific page each tab is loaded to.

This change inverts the order of the title to maintain as much useful context as possible on the tabbar.

Simplified some functions in permissions.py

Should it be that way?

djangorestframework-camel-case in docs

sections of the docs for both parsers and renderers.

Added missing custom validation method test

Updated test class name to be unique

Fix tutorial's step 1 for Python 3

Merge branch 'nullbooleanfield_testcase' of git://github.com/dpetzel/django-rest-framework into dpetzel-nullbooleanfield_testcase

Conflicts:
	rest_framework/tests/test_serializer.py

Fix infered field type for models.NullBooleanField

Fix nested serializers with renamed fields