Gets a authorization token for session reuse.

            http://docs.exosite.com/portals/#get-user-token-for-openid-user

def get_user_token(self):
        """
            Gets a authorization token for session reuse.

            http://docs.exosite.com/portals/#get-user-token-for-openid-user
        """
        headers = { 'User-Agent': self.user_agent(),
                'Host': self.domain(),
                'Accept': '*/*',
        }
        headers.update(self.headers())
        r = requests.get(   self.portals_url()+'/users/_this/token',
                            headers=headers,
                            auth=self.auth())
        if HTTP_STATUS.OK == r.status_code:
            return r.text
        else:
            print("get_user_token: Something went wrong: <{0}>: {1}".format(
                        r.status_code, r.reason))
            r.raise_for_status()