Adds delivery_mode setting support to publishMessage()

Fixes #25 

Adds parameter `$deliveryMode` to publishMessage(). The default 2 (persistent) needs to also support optionally setting to 1 (non-persistent).
```
   * @param int $deliveryMode
   *  1: non-persistent, faster but no logging to disk, ~ 3x
   *  2: persistent, write a log to disk
   */
  public function publishMessage($payload, $deliveryMode = 2) {
```