Allow for updating options

I $observe the attrs for changes and update the options. This enables you to update the options on the fly. 

Example being enabling and disabling the ability to sort, during an "edit" action.

Author: robjames

src/sortable.js

@@ -8,9 +8,11 @@ angular.module('ui.sortable', []).value('uiSortableConfig',{}).directive('uiSort
     return {
       require: '?ngModel',
       link: function(scope, element, attrs, ngModel) {
-        var onReceive, onRemove, onStart, onStop, onUpdate, opts;
+        var onReceive, onRemove, onStart, onStop, onUpdate, opts = {};
 
-        opts = angular.extend({}, uiSortableConfig, scope.$eval(attrs.uiSortable));
+        attrs.$observe('ui-sortable', function(){
+          opts = angular.extend({}, uiSortableConfig, scope.$eval(attrs.uiSortable));
+        });
 
         if (ngModel) {