Implement entity joins

[beikov]

Since all JPA 2.1 providers support entity joins in their latest versions, we can safely support that too.

See

• Hibernate: http://in.relation.to/2016/02/10/hibernate-orm-510-final-release/
• EclipseLink: http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#ON
• Datanucleus: http://www.datanucleus.org/products/datanucleus/jpa/jpql.html