dynamic sort by for page list

a change made for one of our projects by naume http://www.concrete5.org/profile/-/72887/
this allows you to use sortBy in the same way we can use filterBy

```php
$pl = new PageList(); 
$pl->sortByMyAttr('desc');
// $pl->sortBy('my_attr', 'desc');
$pages = $pl->getPage();
```