fix(gridster): serialize returns an Array object, not a jQuery object

Fixes #394

src/jquery.gridster.js

@@ -790,7 +790,7 @@
         return $widgets.map($.proxy(function(i, widget) {
             var $w = $(widget);
             return this.options.serialize_params($w, $w.coords().grid);
-        }, this));
+        }, this)).get();
     };