fix(gridster): destroy resize_api

Call  `resize_api.destroy()`  to fix  memory leak.

Closes #473

src/jquery.gridster.js

@@ -3118,6 +3118,10 @@
             this.drag_api.destroy();
         }
 
+        if (this.resize_api) {
+            this.resize_api.destroy();
+        }
+
         this.remove_style_tags();
 
         remove && this.$el.remove();