Added NestedHyperlinkedModelSerlalizer

This overrides the standard HyperlinkedModelSerializer so the
NestedHyperlinked*Fields are automatically tied to the serializer
when it is generating fields automatically.

It uses internal fields which are under the private API (one which
has not been implemented yet) which is being worked into the public
API.

https://github.com/tomchristie/django-rest-framework/pull/1058