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 agree that it looks strange! I will adapt this part of the documentation in order to make it explicit that 'versions_tests' only needs to be added if tests should be run.