Support for abstract parent classes

We're using a shared abstract parent class for our models, where I added the search_manager for the fts. The problem was that the method contribute_to_class added all needed methods to the abstract class and saved that model internally. At updating a specific instance, there occured the exception that abstract classes don't have a pk-column, which is used to build the sql-query.
I changed the behaviour of contribute_to_class to skip abstract classes, so the methods where assigned to the final derived classes and now everything works fine.