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!
il me semble que le MUST est inutile (et en dessous aussi) car join_path est suffisamment malin pour que

~~~
"/home/jean/wiki/somewiki".join_path("/usr/share/nitiwiki/some/file") == "/usr/share/nitiwiki/some/file"
~~~