Fixed #13252 -- Added ability to serialize with natural primary keys.

Added ``--natural-foreign`` and ``--natural-primary`` options and
deprecated the ``--natural`` option to the ``dumpdata`` management
command.

Added ``use_natural_foreign_keys`` and ``use_natural_primary_keys``
arguments and deprecated the ``use_natural_keys`` argument to
``django.core.serializers.Serializer.serialize()``.

Thanks SmileyChris for the suggestion.