Add MetaDatum::Users model, factory and specs

Squashed commit of the following:

commit e6c68269a8d7f708e7415862e16d1ff4946b449f
Author: Thomas Schank <DrTom@schank.ch>
Date:   Wed Dec 17 15:12:42 2014 +0100

    Minor fixes

commit 7279f91e02cacae941674a91d5066154865b2e00
Author: Thomas Schank <DrTom@schank.ch>
Date:   Wed Dec 17 15:05:29 2014 +0100

    spec fix syntax

commit 602f4690f2e39760149a02c7c2612a194b3383ec
Author: Thomas Schank <DrTom@schank.ch>
Date:   Wed Dec 17 15:03:35 2014 +0100

    introduce meta_datum_users ...