The Web server nitcorn, libevent and others

This PR contains many fixes and a major rework of the Nitcorn project by Jean-Philippe Caissy, Frederic Sevillano and Guillaume Auger. 

Nitcorn is a Web server framework to create sever-side applications in Nit. It is currently used at http://tnitter.xymus.net/ and http://vps.xymus.net/nit/index.html The README files is missing and will be added soon.

What it does do:
* GET and POST support (for small uploads)
* Virtual hosts and routes
* Sessions
* Reading cookies
* Adding virtual hosts (and interfaces) on the fly
* Adding/removing route on the fly
* Play well with existing modules `drop_privileges`, `md5` and `sqlite3`.

What it does not do (yet):
* Encryption
* Support receiving big files
* Writing cookies
* Remove virtual hosts (and interfaces) on the fly

The Tnitter project will be in a future PR.
On gardera ça pour une prochaine PR... L'exemple nitcorn_hello_world serait peut-être mieux pour ça. Le but de file_server_on_port_80 est de montrer comment utiliser nitcorn avec drop_priviledges.