code: public ItemRef once(StorageEvent eventType, final OnItemSnapshot onItemSnapshot) {
  		return once(eventType, onItemSnapshot, null);
  	}
docs: Attach a listener to run the callback only one the event type occurs for this item.
  
  <pre>
  
  StorageRef storage = new StorageRef("your_app_key", "your_token");
  
  TableRef tableRef = storage.table("your_table");
  
  ItemRef itemRef = tableRef.item(new ItemAttribute("your_primary_key_value"),
  new ItemAttribute("your_secondary_key_value"));
  
  itemRef.once(StorageRef.StorageEvent.UPDATE,new OnItemSnapshot() {
  &#064;Override
  public void run(ItemSnapshot itemSnapshot) {
  if(itemSnapshot != null){
  Log.d("ItemRef", "Item updated: " + itemSnapshot.val());
  }
  }
  });
  
  </pre>
  
  @param eventType
  The type of the event to listen. Possible values: put, update, delete.
  @param onItemSnapshot
  The callback to run when the event occurs. The function is called with the snapshot of affected item as argument.
  @return
  Current item reference