Add htacess and icons for fancy indexing.

Add htacess and icons for fancy indexing.

The apache server on people.tuebingen.mpg.de has FancyIndexing via
mod_autoindex enabled. Let's use its features to provide a nicer view
of the download directory.

This adds two .png icons for tarballs and signatures, respectively.
These icons are listed in the new .htaccess file to instruct apache
to show the icons next to the download links, along with a file
description.

On the old page files were listed alphabetically. So one had to scroll
to the bottom to get to the newest tarball link. FancyIndexing lets us
specify the sort order and we use this feature to show new files first.