Use NestedTree trait in favour of NestedSet behavior

You should be using NestedTree trait (recommended) or NestedSetModel behavior, but not both :-)

NestedSetModel behavior is deprecated:

```
/**
 *
 * DEPRECATED WARNING: This class is deprecated and should be deleted
 * if the current year is equal to or greater than 2015.
 * 
 * @todo Delete this file if year >= 2015.
 *
 * See trait: October\Rain\Database\Traits\NestedTree
 *
 */
```
This could fix issue #10