Missing field compatibility rules and migration code.

If you install the application on your Django project and apply migrations, it complains because some fields are missing in South migrations.

Missing fields are the ones created as instances of ListField, and thus South does not account for [them](http://south.readthedocs.org/en/latest/customfields.html#custom-fields).

A [solution](http://south.readthedocs.org/en/latest/tutorial/part4.html#simple-inheritance) is described in South tutorial documentation. 

This patch adds the code and the required missing migration.