Merge: Manual: rewrite the man for nitg

If people like it, I may write the manpage for the other tools.

I generated the markdown to http://nitlanguage.org/tools/nitg/ so you could see the rendered result in HTML.
I plan to have pages for the tools automatically synchonized from the files in /share/man.

Pull-Request: #871
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>