Bizou now handles correctly data and images directories. One can now
configure
* `MEDIA_BASE_DIR`: base directory for `images` and `data` and
* `{IMAGES,DATA}_URL`: the base url that should be used to point to a
file in `IMAGES_DIR` (resp. `DATA_DIR`).
Assuming the web server itself can correctly serve those files at the
given URL and Bizou has the correct permissions for those locations,
Bizou should be able to work as well as using relative paths.
