Manual: rewrite the man for nitg

If people like it, I may write the manpage for the other tools.

I copy-pasted 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.
If we follow the format of other manpages, it should be the following:
> nitg — compiles Nit programs