Add proper South introspection rules, fixing db_type for migrations.

[fletom]

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')),

[derek-schaefer]

Thanks for the fix!