Add trait: IteratorAggregate

# Usage

```php
<?php

namespace App\Some\Namespace;

use Samurai\Samurai\Component\App\IteratorAggregate;

class Iterator implements \IteratorAggregate
{
    use IteratorAggregate;
}
```

```php
$iterative = new Iterator();

$iterative->add('attribute 1');
$iterative->add('attribute 2');
$iterative->add('attribute 3');

foreach ($iterative as $i) {
  echo $i;
}

// attribute 1
// attribute 2
// attribute 3
```