In callback url: http://host/callback?code=123&state=xyz

        use code and state to get an access token.

def get_access_token(self, code, state=None):
        '''
        In callback url: http://host/callback?code=123&state=xyz

        use code and state to get an access token.
        '''
        kw = dict(client_id=self._client_id, client_secret=self._client_secret, code=code)
        if self._redirect_uri:
            kw['redirect_uri'] = self._redirect_uri
        if state:
            kw['state'] = state
        opener = build_opener(HTTPSHandler)
        request = Request('https://github.com/login/oauth/access_token', data=_encode_params(kw))
        request.get_method = _METHOD_MAP['POST']
        request.add_header('Accept', 'application/json')
        try:
            response = opener.open(request, timeout=TIMEOUT)
            r = _parse_json(response.read())
            if 'error' in r:
                raise ApiAuthError(str(r.error))
            return str(r.access_token)
        except HTTPError as e:
            raise ApiAuthError('HTTPError when get access token')