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).
.... and to continue on the same topic it definitely looks strange from a user point of view to installed 'versions_tests' app. 
It might be ok to do this the first minutes and for the user to run the test to check that everything seems alright. But afterwards he should just declare 'versions' in the INSTALLED_APPS.