Changes:
- Fix def __on_error()
- Delete credentials (login, password) from object after successful auth
- get_token() return token as string
- save_to_config() take argument for filename of config
- auth() have 4 NEW handlers:
- on_captcha()
- on_2fa()
- on_invalid_client()
- on_critical_error()
- add simple default realise of 4 handlers auth()