Created new file passwd_reader.py.

Moved:
    api.set_permissions() -> passwd_reader.set_permissions()
    api._save_users() -> passwd_reader.save_users()
    api._load_users() -> passwd_reader.load_users()

Bump #11, where it will be used later.

Fixed a lot of bugs (I've installed pylinter, so now I see a lot more).
LEAVE_BAD_FILES changed to True by default.