Fetch the token, using the verification code. Also, make sure the state
        received in the response matches the one in the request. Returns the
        access_token.

def fetch_token(self, code, state):
        """
        Fetch the token, using the verification code. Also, make sure the state
        received in the response matches the one in the request. Returns the
        access_token.
        """
        if self.state != state:
            raise MismatchingStateError()
        self.token = self.oauth.fetch_token(
            '%saccess_token/' % OAUTH_URL, code=code,
            client_secret=self.client_secret)
        return self.token['access_token']