code: public TableRef on(StorageEvent eventType, final OnItemSnapshot onItemSnapshot) {
  		return on(eventType, onItemSnapshot, null);
  	}
docs: Attach a listener to run every time the eventType occurs.
  
  <pre>
  StorageRef storage = new StorageRef("your_app_key", "your_token");
  
  TableRef tableRef = storage.table("your_table");
  
  // Add an update listener
  tableRef.on(StorageRef.StorageEvent.UPDATE, new OnItemSnapshot() {
  &#064;Override
  public void run(ItemSnapshot itemSnapshot) {
  if (itemSnapshot != null) {
  Log.d("TableRef", "Item updated: " + itemSnapshot.val());
  }
  }
  });
  
  </pre>
  
  @param eventType
  The type of the event to listen. Possible values: put, update, delete
  @param onItemSnapshot
  The function to run whenever the event occurs. The function is called with the snapshot of affected item as argument.  If the event type is "put", it will immediately trigger a "getItems" to get the initial data and run the callback with each item snapshot as argument.  Note: If you are using GCM the value of received ItemSnapshot can be null.
  @return Current table reference