Fixes for JPA embeddables (issue 85),  enums (issue 133) and $expand (issue 219)

Issue 85: Provide support for @Embeddable/@Embedded attributes for non-key types.

Issue 133: JPAProducer: enums not suported.
Enums are given as strings, just as the metadata already describe them in the original code before this fork. Tested in both reading and writing scenarios.

Issue 219: $expand broke in version 0.7 with JPAProducer.
Version 0.6 worked, but it filled the relationships using lazy loading, resulting in many database roundtrips and big performance degradation. This proposed implementation not only reenables the feature but it also creates JPA queries which fetch the expanded relationships eagerly in a single roundtrip.