Migration awareness

This PR makes CleanerVersion aware of migrations. A Django application including CleanerVersion-Versionables in its models can now be migrated. 
It also resolves issue #22, which comes down to wrongly generated SQL-statements due to incorrectly guessed relationship table fields (by Django).
That way we are sure to have some working models in place and covering every possible combination of relationships, for which migration files should be created.