adding a simulation mode

[andrea crotti]

One thing that I don't quite understand is why some calls even if I
catch the exception still makes the whole program quit.
For example this

    try:
        copytree('sjkdf', 'dsflkj')
        Popen(['notfouhd'], shell=True)
    except Exception as e:
        print("here")


behaves differently from:

    try:
        Popen(['notfouhd'], shell=True)
        copytree('sjkdf', 'dsflkj')
    except Exception as e:
        print("here")

because if copytree fails it quits anyway.
I also looked at the code but can't quite get why.. any idea?