code: public ItemRef incr(final String property, final OnItemSnapshot onItemSnapshot, final OnError onError){
  		return this.incr(property, null, onItemSnapshot, onError);
  	}
docs: Increments by one a given attribute of an item. If the attribute doesn't exist, it is set to zero before the operation.
  
  <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.incr("your_property",new OnItemSnapshot() {
  &#064;Override
  public void run(ItemSnapshot itemSnapshot) {
  if(itemSnapshot != null){
  Log.d("ItemRef", "Item incremented: " + itemSnapshot.val());
  }
  }
  }, new OnError() {
  &#064;Override
  public void run(Integer integer, String errorMessage) {
  Log.e("ItemRef", "Error incrementing item: " + errorMessage);
  }
  });
  
  </pre>
  
  @param property The name of the item attribute.
  @param onItemSnapshot The callback invoked once the attribute has been incremented successfully. The callback is called with the snapshot of the item as argument.
  @param onError The callback invoked if an error occurred. Called with the error description.
  @return Current item reference