function onDragEvent(evt)
		{
			element.addEventListener("dragexit", onDragEvent);
			element.addEventListener("dragover", onDragEvent);
			element.addEventListener("drop", onDrop);
			evt.stopPropagation();
			evt.preventDefault();
			if(evt.type == "dragenter" && callback_enter)
				callback_enter(evt, this);
			if(evt.type == "dragexit" && callback_exit)
				callback_exit(evt, this);
		}
Helper, makes drag and drop easier by enabling drag and drop in a given element
@method createDropArea
@param {HTMLElement} element the element where users could drop items
@param {Function} callback_drop function to call when the user drops the item
@param {Function} callback_enter [optional] function to call when the user drags something inside