Add or_where case to _parse_where_array()

This change makes the following forms possible.

```php
$items = \Model_Item::find('all', array(
    'where' => array(
        array(
            'or:0' => array('title', 'like', '%foo%'),
            'or:1' => array('description', 'like', '%foo%'),
        ),
        array(
            'or:0' => array('title', 'like', '%bar%'),
            'or:1' => array('description', 'like', '%bar%'),
        )
    )
));
```