Validate application ID and request is from Alexa.

def request(self, app_id=None, body=None, stamp=None, url=None, sig=None):
        """Validate application ID and request is from Alexa."""
        if self.app_id:
            if not self.application_id(app_id):
                return False

        if (url or sig):
            if not (body and stamp and url and sig):
                raise ValueError('Unable to validate sender, check arguments.')
            else:
                if not self.sender(body, stamp, url, sig):
                    return False

        return True