code: def get_access_token(self, code):
          
          payload = {'grant_type': 'authorization_code',
                     'code': code,
                     'client_id': self._client_id,
                     'client_secret': self._client_secret,
                     'redirect_uri': self._redirect_uri,}
          req = requests.post(settings.API_ACCESS_TOKEN_URL, data=payload)
          data = req.json()
          return data.get('access_token')
docs: Returns Access Token retrieved from the Health Graph API Token 
          Endpoint following the login to RunKeeper.
          to RunKeeper. 
          
          @param code: Code returned by Health Graph API at the Authorization or
                       RunKeeper Login phase.
          @return:     Access Token for querying the Health Graph API.