Allow linking of two sortable list

It's a simple feature. It allows to use the jQuery UI linking of two sortable lists like so:

Controller:
```javascript
function TestCtrl($scope) {
  $scope.items1 = ['Item1', 'Item2', 'Item3'];
  $scope.items2 = [];
}
```

View:
```html
<ul ui-sortable ui-sortable-connect=".group" ng-model="items1" class="group">
  <li ng-repeat="item in items1">{{ item }}</li>
</ul>
<ul ui-sortable ui-sortable-connect=".group" ng-model="items2" class="group">
  <li ng-repeat="item in items2">{{ item }}</li>
</ul>
```