Feature to dump payload of files using manage.py dumpdata

This is a reference implementation, which allows to dump the files payload together with its meta-data, when running ``manage.py dumpdata > dumpfile.json``.

A dumpfile which is reimported using ``manage.py loaddata dumpfile.json`` restores the file on disk.
This simplifies backups and migrations of projects using **django-filer** to manage their media data.

More details here: https://github.com/jrief/django-filer/blob/serialize-payload/docs/dump_payload.rst

This feature has been discussed on the mailing list in December 2012. It is fully backward compatible with existing dumps.