Add MediaEntryUserpermission table, model, spec and data-migration

Squashed commit of the following:

commit d3aae26123f133d32ba3090dbff24ea7e214dcf6
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Nov 21 08:59:26 2014 +0100

    Add spec for MediaEntryUserpermission

commit 373bf5dd00cceb641ccf5afb548f172de6f956cf
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Nov 21 08:28:55 2014 +0100

    Add media_entry_userpermission model

commit 64be79bda1edab0b23000b26d84edba8e33303f4
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Nov 21 08:03:07 2014 +0100

    Create table media_entry_userpermissions