gridster: new method remove_all_widgets

Remove multiple widgets without chaining callbacks was not possible so I've added a new argument to remove_widget method named `silent` which if it's value is true, widgets below the removed will not move up.

Also added the method remove_all_widgets.

Fixes #63

src/jquery.gridster.js

@@ -433,12 +433,21 @@
     *
     * @method remove_widget
     * @param {HTMLElement} el The jQuery wrapped HTMLElement you want to remove.
+    * @param {Boolean|Function} silent If true, widgets below the removed one
+    * will not move up. If a Function is passed it will be used as callback.
+    * @param {Function} callback Function executed when the widget is removed.
     * @return {Class} Returns the instance of the Gridster Class.
     */
-    fn.remove_widget = function(el, callback) {
+    fn.remove_widget = function(el, silent, callback) {
         var $el = el instanceof jQuery ? el : $(el);
         var wgd = $el.coords().grid;
 
+        // if silent is a function assume it's a callback
+        if ($.isFunction(silent)) {
+            callback = silent;
+            silent = false;
+        }
+
         this.cells_occupied_by_placeholder = {};
         this.$widgets = this.$widgets.not($el);
 
@@ -449,9 +458,11 @@
         $el.fadeOut($.proxy(function() {
             $el.remove();
 
-            $nexts.each($.proxy(function(i, widget) {
-                this.move_widget_up( $(widget), wgd.size_y );
-            }, this));
+            if (!silent) {
+                $nexts.each($.proxy(function(i, widget) {
+                    this.move_widget_up( $(widget), wgd.size_y );
+                }, this));
+            }
 
             this.set_dom_grid_height();
 
@@ -462,6 +473,22 @@
     };
 
 
+    /**
+    * Remove all widgets from the grid.
+    *
+    * @method remove_all_widgets
+    * @param {Function} callback Function executed for each widget removed.
+    * @return {Class} Returns the instance of the Gridster Class.
+    */
+    fn.remove_all_widgets = function(callback) {
+        this.$widgets.each($.proxy(function(i, el){
+              this.remove_widget(el, true, callback);
+        }, this));
+
+        return this;
+    };
+
+
     /**
     * Returns a serialized array of the widgets in the grid.
     *