Make human task service work with OpenJPA

I patched human-task-service-core to make it work with OpenJPA. I do not include the full patch here but I will if someone is interested.

This pull request only shows the first step I had to do: OpenJPA (JPA in generell?) does not allow to use AUTO generation type together with a custom sequence generator. I therefore removed the generator.

This seams to work well in OpenJPA and Hibernate.