Add document date to listing block if ftw.file is installed.

We need to be able to display `Document Date` in the listing block.
The `Document Date` is a custom field defined by `ftw.file` (which also registers an index and metadata).

This PR makes the `Document Date` available if `ftw.file` is installed.

![bildschirmfoto 2013-09-10 um 11 18 54](https://f.cloud.github.com/assets/7469/1113336/0b4afe80-19fa-11e3-8db7-22d0fd4d8b4f.png)

@elioschmutz please review
/cc @bbuehlmann