Headless setup under apache2 / ubuntu 12.04

I operate recoll in a headless setup. For that I had to do a couple of tweaks. Most importantly the changes in webui.py. Without them I got errors using apache2/wsgi.

The setups features are:
- Runs on a dedicated ubuntu 12.04 server
- Indexes files on a NAS
- Makes files available for search for several users in the local network
- Files are served by an apache2 server using wsgi. That way I can protect the content using authentication / TLS.

The changes in this pull request are:
- solve error under wsgi: UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 26: ordinal not in range(128)
- allow having recoll under an arbitrary subdirectory
- added documentation for headless setups using wsgi
- make download the default file fetch behaviour