Add proper South introspection rules, fixing db_type for migrations.

Since South didn't know that it had to include the `db_type` argument when initializing the field, the actual `db_type` value that got written to the database was always the same as TextField. In Postgres, this would be `text` even if you passed `json`. (It only worked correctly if you were using `syncdb` instead.)

Now South correctly adds the right arguments when creating migration files:

    ('data', self.gf('json_field.fields.JSONField')(default='null', db_type='json')),