plugin XEP-0334: fixed and improved message processing hints: - wrong elements were used in this plugging, it could not work properly - use constants for hints - add addHint method so other plugins can add a hint easily - hints can be added by frontends in mess_data['extra']['hints'] (serialized dict) or by other plugins using addHint - if history is skipped on message reception, mess_data['extra']['history']['skipped'] is set - introduced PLUGIN_INFO['usage'] for usage information for developers