Merge branch 'validator-pollution' of git://github.com/mhsparks/django-rest-framework into mhsparks-validator-pollution

Add allow_blank for ChoiceField #2184

This makes a ChoiceField optional in HTML if
model field has `blank=True` set.

Use httpie for tutorials

Added ListSerializer.validate().

Add misc note about localization

Cosmetic fixes in the API browser HTML.

Amend ViewSet docs to warn of potential problem

Fix missing validated_data in `raise_errors_on_nested_writes` (#2221)

I went through this exact problem and so thought the docs might
benefit from a small hint at the appropriate point.

Update the tutorial against DRF v3 (part 4 & 5)

PrimaryKeyRelatedField now needs a queryset argument.
urls now don't use urlpatterns.

Raise error if `fields` on serializer is not a list of strings.

Better serializer errors for nested writes.