Exchange a temporary OAuth2 code for an access token.

        Param: code -> temporary OAuth2 code from a Pushed callback
        Returns access token as string

def access_token(self, code):
        '''Exchange a temporary OAuth2 code for an access token.

        Param: code -> temporary OAuth2 code from a Pushed callback
        Returns access token as string
        '''
        parameters = {"code": code}
        access_uri = "/".join([BASE_URL, API_VERSION, ACCESS_TOKEN])
        # RFC non-compliant response prevents use of standard OAuth modules
        success, response = self._request(access_uri, parameters)
        if success:
            return response['response']['data']['access_token']
        else:
            raise PushedAPIError(
                response['error']['type'],
                response['error']['message']
            )