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.
Ca peut etre la a fois un exemple et un programme utilisable.
Déplace-le dans la lib et parle-en dans le README.

Pour les modifs que je demande. c'est juste un truc du genre:

~~~
var opt_dir = new OptionString("The root directory served", "-d", "--dir")
var opt_host = new OptionString("The hostname", "-H", "--host")
opt_host.default_value = "localhost"
var opt_port = new OptionString("The port", "-p", "--port")
opt_port.default_value = 80
# ...
var vh = new VirtualHost(opt_host.value + ":" + opt_port.value.to_s)
vh.routes.add new Route(null, new FileServer(opt_dir.value))
~~~