Include ftw.zipexport

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

## Changes:
* Included and activated ftw.zipexport.
* Configured ftw.zipexport to restrict export on dossiers.

![screen shot 2013-10-02 at 12 29 26](https://f.cloud.github.com/assets/1375745/1252564/c9569092-2b4d-11e3-9fb8-a64bc48477f1.png)
![screen shot 2013-10-02 at 12 29 59](https://f.cloud.github.com/assets/1375745/1252567/d2789fd0-2b4d-11e3-8f11-2443fc84f2a6.png)
![screen shot 2013-10-02 at 13 22 36](https://f.cloud.github.com/assets/1375745/1252826/ffb82a22-2b54-11e3-8c05-61b253b37b20.png)