Add autologin command and miscellaneous fixes.

Changes in api.py:
- payload variable is splitted in two variable (username and password) in _obtain_user_token method.
- username is now stored as self._user_name in _obtain_user_token method.
- get_mix_liked method renamed as get_liked_mixes.
- self.user_token marked as private.
- Modify get_liked_mixes docs.

Changes in client.py:
- Add do_autologin method to toggle autologin on|off.
- Refactored autologin in preloop to use do_login method.
- Fix do_login to allow password with spaces.
- Add ConnectionError exception to do_login method.
- Renamed get_logged method as get_login_status.
- Renamed self.logged variable as self._logged_in.
- Renamed do_like method as do_liked_mixes.

Changes in README.rst:
- Modified command name subsquently to method renaming.
- Added autologin command description.