Enable retrieving sub-attributes from 'cas:attributes' in cas server response.

For instance, with this server response :
```xml
<cas:serviceResponse xmlns:cas='http://www.example.com/cas'>
	<cas:authenticationSuccess>
		<cas:uid>12345678</cas:uid>
		<cas:user>ext-cturpin</cas:user>
		<cas:attributes>
			<cas:attribute name="mail" value="chturpin@example.com"/>
			<cas:attribute name="displayName" value="ext-Charles-Henri Turpin"/> 
		</cas:attributes>
	</cas:authenticationSuccess>
</cas:serviceResponse>
```
The attributes array will now contains `mail` and `displayName` keys.