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).
Why the test models moved away from the test file? I think it was a bit more practical to have them located in the same file.

I think you did this so that the test_commands has some models to work with. But maybe we could just have created some specific models for the test_commands and leave the others in their original test file.