Merge: Parallelization phase introduction

Introduces the begining of the parallelization in Nit !

For now, only accepts the "threaded" annotation on methods without return value and without parameters.

Coming next :
Parametrized methods,
methods with return value,
for loops.

Pull-Request: #1247
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>