Dokumenten-Tab mit Öffentlichkeitsstatus erweitern

Anpassungen an TabbedView Konfiguration
 - [x] "public trial"-Spalte in Dokumenten-Tab hinzufügen
https://github.com/4teamwork/opengever.core/blob/master/opengever/tabbedview/browser/tabs.py#L177
 - [x] Werte übersetzt darstellen: https://github.com/4teamwork/opengever.core/blob/master/opengever/task/helper.py

 - [x] Add IClassification on mail

Index und Metadaten hinzufügen:
http://docs.plone.org/develop/plone/searching_and_indexing/indexing.html
 - [x] Neuer Katalog-Index `public_trial` index in [indexes.py](https://github.com/4teamwork/opengever.core/blob/master/opengever/document/indexers.py#L107) 
 - [x] Katalog Metadata `public_trial` metadata in [catalog.xml](https://github.com/4teamwork/opengever.core/blob/master/opengever/document/profiles/default/catalog.xml) eintragen (muss reindexiert werden) 
 - [x] Custom Sorter [opengever.tabbedview.tabs](https://github.com/4teamwork/opengever.core/blob/master/opengever/tabbedview/browser/tabs.py#L132)
  * Upgrade-Step
    - [x] `public_trial` index [neu rechnen] (https://github.com/4teamwork/ftw.upgrade#upgrade-step-helpers) `catalog_reindex_objects` (mit logger)
    - [x] `public_trial` metadata per profil (catalog.xml) hinzufügen.

Please be a little bit more specific in your commit messages or changelog entries.