code: public Future<OAuth2AccessToken> getAccessToken(AccessTokenRequestParams params,
              OAuthAsyncRequestCallback<OAuth2AccessToken> callback) {
          return sendAccessTokenRequestAsync(createAccessTokenRequest(params), callback);
      }
docs: Start the request to retrieve the access token. The optionally provided callback will be called with the Token
  when it is available.
  
  @param params params
  @param callback optional callback
  @return Future