BaseAttribute should be annotated as MappedSuperclass

Hi,

Post 2.x platform is made compatible with Hibernate 4. Hence, entities can now be annotated to avoid xml mappings. I can see that properties in BaseOpenmrsMetadata and BaseOpenmrsData have been annotated. However, I don’t see any annotations for BaseAttribute and BaseCustomizableData; neither are these two annotated as @MappedSuperclass.

I suggest making BaseAttribute as MappedSuperclass and annotate its properties.

Related discussion on [Openmrs Talk|https://talk.openmrs.org/t/handling-jpa-annotations-for-baseattribute/17873.]