Support for @MappedSuperclass and @Inheritance when validating and read metadata for @Id

This change partly resolves #55 and #56 in the terms when it is required to use @MappedSuperClass and to have an @Id field in it (it just reads the fields from the entity class hierarchy instead of reading them from the main entity class directly).
I'm not sure if you are going to merge this request, but these issues are really blockers for us, so we have to use our own fixed Kundera version temporarily.