Register library device id and get initial device list.

async def register(self):
        """Register library device id and get initial device list. """
        url = '{}/Sessions'.format(self.construct_url(API_URL))
        params = {'api_key': self._api_key}

        reg = await self.api_request(url, params)
        if reg is None:
            self._registered = False
            _LOGGER.error('Unable to register emby client.')
        else:
            self._registered = True
            _LOGGER.info('Emby client registered!, Id: %s', self.unique_id)
            self._sessions = reg

            # Build initial device list.
            self.update_device_list(self._sessions)

            asyncio.ensure_future(self.socket_connection(), loop=self._event_loop)