DATAJPA-445 - Enable constructor injection for EntityManagers.

Enabling repositories now registers a BeanFactoryPostProcessor that will register a SharedEntityManagerCreator BeanDefinition for all EntityManagerFactory definitions available in the ApplicationContext.

We register the bean name of the EMF as qualifier for the BeanDefinition for the EntityManager to allow an explicit reference in multi-EMF scenarios.

Renamed default persistence unit to spring-data-jpa.