Include ftw.zipexport as 

The package [ftw.zipexport](https://github.com/4teamwork/ftw.zipexport) should be integrated in to opengever.core.

In the first version i would propose the following functionalities should be possible:
 - Exporting a complete dossier or subdossier over a `object_buttons` action
 - Exporting document and mails from all document listing by selecting objects and use a corresponding tabbedview action:
   - Document tab in dossier
   - Document tab in tasks
   - MyDocuments

Todo:
 - Add corresponding dependency to `opengever.policy.base`
 - Configure tab-actions
 - Activate marker behavior for the specific contents


@shylux