Merge: introduce nit_env.sh to setup the shell environement

Merge: introduce nit_env.sh to setup the shell environement

The script `nit_env.sh` tries to auto-magically configure PATH, MANPATH and bash completion for users. The point is to be as portable and simple a possible for the final used has he just has to write

~~~
$ source misc/nit_env.sh
~~~

and get a working setup.

Moreover, if `install` in given as argument, then the script register itself to the user `$HOME/.profile`.

~~~
$ source misc/nit_env.sh install
~~~

One advantage is that the script invocation is registered in `.profile`, thus future evolutions of the script will be automatically used in future sessions of the users.

Pull-Request: #1784
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>