Allow for missing non-required nested objects.

Serializer fields which are themselves serializers should not be required.
Specifically, if a nested object is set to "required=False", it should be
possible to serialize the main object and have the sub-object set to None/null.
Merge pull request #831 from brianz/master

Allow nested fields to be missing
Added @craigds, @pyriku, @brianz - Yay for sprints!
Initial commit of fix for https://github.com/tomchristie/django-rest-framework/issues/775
Merge branch 'master' of git://github.com/tomchristie/django-rest-framework
documentation for https://github.com/tomchristie/django-rest-framework/issues/775
added comment about error behavior with DEBUG=True
Added @grimborg and @ryankask. Go Team. :)
when source='*' on a nested serializer, expand fields into outer serializer when writing. fixes #765
Merge
Merge pull request #856 from grimborg/http_debug_error_page_doc

added comment about error behavior with DEBUG=True
Tweak docs
Merge branch 'master' of https://github.com/mmcardle/django-rest-framework into styling-fixes
Styling tweaks
Merge pull request #859 from craigds/master

fix for #765