Nitiwiki: a quick wiki builder based on markdown files

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, entirely based on nitiwiki.