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).
I don't really understand why we don't want to import 'versions' app any longer.
I understand why it works though but I think it is weird to have 'versions_tests' in one's INSTALLED_APPS and not 'versions' also.

It works, I think, because 'versions_tests' models.py file includes the necessary models from versions. So it is like loading the app 'versions' by transitivity (through 'versions_tests'). But that looks dodgy, don't you think?