Added an override for support for jQuery

Gridster's `add_widget` ultimately passes on the first parameter to
jQuery, and jQuery accepts either a string, a DOM element, or a
jQuery object.

`add_widget`'s first parameter should reflect what jQuer supports.

jquery.gridster/gridster.d.ts

@@ -184,6 +184,11 @@ interface Gridster {
 	**/
 	add_widget(html: HTMLElement, size_x?: number, size_y?: number, col?: number, row?: number): JQuery;
 
+	/**
+	* @see add_widget
+	**/
+	add_widget(html: JQuery, size_x?: number, size_y?: number, col?: number, row?: number): JQuery;
+
 	/**
 	* Change the size of a widget.
 	* @param $widget The jQuery wrapped HTMLElement that represents the widget is going to be resized.