Attribute suppport

This pull request adds support for XML attributes

Example XML:
```xml
<books>
  <book author="Test">Book #1</book>
  <book>Book #2</book>
</books>
```

Example PHP:
```php
$books= $address->next(new Enclosing('/'))->next(new ArrayOf(new CreationOf('util.address.unittest.Book', [
  '.'       => function($iteration) { $this->name= $iteration->next(); },
  '@author' => function($iteration) { $this->author= new Author($iteration->next()); }
])));
```