Enhancement: Add Auxiliary Methods for Migrations to Reference Inexistent Columns #137

As originally discussed here https://groups.google.com/forum/#!topic/activate-persistence/EAvvCCRw3QA

Once a column has been removed from the model class, it can no longer be referenced, either by earlier migrations, or within the migration that is removing it (which would be needed if the data was being translated or re-purposed, or simply moved elsewhere.)