Nitiwiki: a quick wiki builder based on markdown files

I talked about that a long time ago, tonight's the night!

This PR introduces `nitiwiki` a contrib tool that generate an html wiki structure based on markdown files.
Behavior is very similar to ikiwiki (but simplified).

Features:

* automatic wiki structure from folders hierarchy
* automatic site menu
* automatic sitemap
* automatic summaries
* easy and rapid templating
* customizable section templates and menus
* rsync synchronization
* git synchronization

See http://moz-code.org/nitiwiki/ for online documentation. This is actually the `README.md` file rendered by nitiwiki. The full wiki can be found in `contrib/examples/nitiwiki`

Another example can be found at http://moz-code.org/uqam/. This is my TA site, entierely based on nitiwiki.

Please disregard commits before #6d73397 since they belongs to #747 and #764.

@privat no excuses, we dump ikiwiki as soon as this PR is merged!
Euh.. quelle est la différence entre mettre un fichier quelconque (genre une image) dans les assets ou dans une section? C'est copié dans `out` dans les deux cas?