DATAJPA-444 - Improve detection of PersistenceProvider implementations.

Since the location of the Hibernate EntityManager implementation changed in Hibernate 4.3 to org.hibernate.jpa.HibernateEntityManager, we now support org.hibernate.jpa.HibernateEntityManager as well as  org.hibernate.ejb.HibernateEntityManager as a Hibernate PersistenceProvider.

Original pull request: #55.