using cache keys as ArrayIterator keys

With this patch we can access to Item by key:
```php
$pool = new Stash\Pool();
$result = $pool->getItemIterator(array('TEST1', 'TEST2'));
echo $result['TEST2']->get();
```

P.S. previous pull request (https://github.com/tedivm/Stash/pull/136) I closed.