Issue 133: Adds enum support as string fields during reads. Note that
before the change the enum fields are already described as strings in
the metadata.
Issue 85: Support for @Embeddable/@Embedded attributes for non-key
types.
Issue 219: Fix $expand for JPAProducer which has been broken since
version 0.6. Version 0.6 worked, but it filled the relationships using
lazy loading, resulting in many database roundtrips and big performance
overhead. This commit creates JPA queries which fetch the expanded
relationships eagerly in a single roundtrip (fetch join).