Get authentication token.  Uses POST to /auth interface.

        :Returns: (str) Authentication JWT

def auth_token(self, apikey=None, secret=None, email=None, password=None):
        """
        Get authentication token.  Uses POST to /auth interface.

        :Returns: (str) Authentication JWT
        """
        if (apikey and secret) or (self._apikey and self._secret):
            body = {
                "key_id": apikey or self._apikey,
                "secret": secret or self._secret
            }
        elif (email and password) or (self._email and self._password):
            body = {
                "user" : email or self._email,
                "password" : password or self._password
            }
        else:
            raise Value("No authentication provided.")
        response = self._post(url.auth, body=body)

        self._check_response(response, 200)
        return self._create_response(response).get("jwt")