List data sources of a portal device with rid 'device_rid'.

            http://docs.exosite.com/portals/#list-device-data-source

def list_device_data_sources(self, device_rid):
        """
            List data sources of a portal device with rid 'device_rid'.

            http://docs.exosite.com/portals/#list-device-data-source
        """
        headers = {
                'User-Agent': self.user_agent(),
        }
        headers.update(self.headers())

        r = requests.get(   self.portals_url()+'/devices/'+device_rid+'/data-sources', 
                    headers=headers, auth=self.auth())
        if HTTP_STATUS.OK == r.status_code:
            return r.json()
        else:
            print("Something went wrong: <{0}>: {1}".format(
                        r.status_code, r.reason))
            return None