fix(gridster): remove_widget Returns the instance of the Gridster Class

Following same pattern as remove_all_widgets --> remove_widget also
return *this* (as per inlined comment documentation)

Author: stefek99

src/jquery.gridster.js

@@ -479,6 +479,8 @@
                 callback.call(this, el);
             }
         }, this));
+
+        return this;
     };