Allow @Id on @ManyToOne fields

Sometimes, a @ManyToOne field has to be the Primary Key, or part of the Primary Key. Adding @Id to the @ManyToOne does not help, an additional property with duplicates the referenced id is needed:

(userId and User field in the Phonenumber class - to be able to set @Id on userId):
http://pastebin.com/d51e021e2

Allowing @Id on @ManyToOne fields (which would make the JoinColumn a PK) would help here. Any maybe this would also fix DDC-116