Clean inconsistent data: a set is never a cover

* Remove auto cover setting
* Debug media_resource_arcs_controller
Add doc/diagrams/Schema_vision.graffle
Add simplecov gem; initialize coverage; attach coverage output to ci
Close open ftp connections during import
Rename ldap_ to institutional_group_

Squashed commit of the following:

commit 07ac25998b53fc60ed2bccc8a982edd56782f2d9
Author: Thomas Schank <DrTom@schank.ch>
Date:   Mon Oct 6 10:15:48 2014 +0200

    Rename ldap_x to institutional_group_x in code

commit 4e3c3d681e93d967f2b1e4180a8ba639c479f059
Author: Thomas Schank <DrTom@schank.ch>
Date:   Mon Oct 6 09:59:54 2014 +0200

    Rename ldap_x to institutional_x in groups table
Admin interface: fix setting special sets