Fix #90: default=None results in "DEFAULT None", and also add delete_primary_key as an alias.
Fix #90: default=None results in "DEFAULT None", and also add delete_primary_key as an alias.

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extra : convert_revision : svn%3A69d324d9-c39d-4fdc-8679-7745eae9e2c8/trunk%40154

nimnull/django-south,philipn/django-south,RaD/django-south,RaD/django-south,nimnull/django-south,RaD/django-south,philipn/django-south