Return new Command object that will be run in specified
        directory.

            new_cmd = old_cmd.in_dir("/usr")

def in_dir(self, directory):
        """
        Return new Command object that will be run in specified
        directory.

            new_cmd = old_cmd.in_dir("/usr")
        """
        new_command = copy.deepcopy(self)
        new_command._directory = str(directory)
        return new_command