Cogito: Cleanup documentation building and add quick reference

Hi,

Here are a few patches that makes it possible to build documentation as
PDF using docbook2pdf (chosen because xmlto doesn't seem to be able to
generate PDFs).

All the patches leads up to adding a quick reference with often used
Cogito commands and associated option. It is inspired by svn-ref and the
goal is to have the PDF version under two pages long. The grouping of
commands could need some more comments.

The quick reference can be translated into HTML, PDF and cg-ref(7)
manpage. The last one is achieved by stripping much of the markup from
the 'default' version since manpages cannot have table layout. Note the
HTML and PDF versions are generated as quick-reference.{html,pdf}. We
might want to generate cg-ref.{html,pdf} instead.

-- 
Jonas Fonseca