Added  "toArray" function

I designed to work seamlessly with the [nested sets](http://en.wikipedia.org/wiki/Nested_set_model) model

#### Usage:
```javascript
$('#list').nestable('toArray');
```

#### Results:
```
'1' ...
        'item_id' => "1"
        'parent_id' ...
        'depth' => "1"
        'left' => "2"
        'right' => "7"
    '2' ...
        'item_id' => "2"
        'parent_id' => "1"
        'depth' => "2"
        'left' => "3"
        'right' => "6"
    '3' ...
        'item_id' => "3"
        'parent_id' => "2"
        'depth' => "3"
        'left' => "4"
        'right' => "5"
    '4' ...
        'item_id' => "4"
        'parent_id' ...
        'depth' => "1"
        'left' => "8"
        'right' => "9"
    '5' ...
        'item_id' => "5"
        'parent_id' ...
        'depth' => "1"
        'left' => "10"
        'right' => "21"
```