code: public static synchronized SailthruClient getInstance(String apiKey, String apiSecret) {
          if (_instance == null) {
              _instance = new SailthruClient(apiKey, apiSecret, DEFAULT_API_URL);
          }
          return _instance;
      }
docs: Synchronized singleton instance method using default URL string
  @param apiKey Sailthru API key string
  @param apiSecret Sailthru API secret string
  @return singleton instance of SailthruClient
  @deprecated