ng-sortable does not wait ($watch) for $scope.ngSortable

[brennancheung]

When items is undefined at the time the template is loaded the directive does not work.

This is very common when the items are populated from a REST API call.

Ex:

In controller:
setTimeout(function(){ $scope.items = [1,2,3]; }, 5000);

In HTML:
<div ng-sortable="items">...</div>

ng-sortable should implement a $watch for the $scope.ngSortable and only run the current linking code when it is defined and an array.

[schartier]

Please make a pull request. It would be appreciated, by me and others :)