add base_classes arg for inheritation to schema_template generation

Hi Peter,
first thanks for you work. It saves a lot of time of me :)
I need an option, to let generated schema class inherit from Rose::DB::Object::Helpers.
This patch does the work. But its only a POC. You don't need to merge. Do it your way.

Excuse, my vim eleminates trailing whitespaces.
Interesting line is 833
Cheers