add batch events method

according to the https://docs.prediction.io/datacollection/eventmodel/#3-batch-events-to-the-eventserver
code tested, the code works in my 0.9.5 environment

Author: ice6

src/predictionio/EventClient.php

@@ -249,6 +249,20 @@ public function createEvent(array $data) {
     return $this->sendRequest('POST', $this->eventUrl, $json);
   }
 
+  /**
+   * Create batch events
+   *
+   * @param array An array describing the events
+   *
+   * @return string JSON response
+   * 
+   * @throws PredictionIOAPIError Request error
+   */
+  public function createEvents(array $data) {
+    $json = json_encode($data);
+    return $this->sendRequest('POST', '/batch' . $this->eventUrl, $json);
+  }
+  
   /**
    * Retrieve an event
    *