Added Option for Multi-Property-Labels

Added the Option to generate a label from a closure within the Proxy-Class. In my work environment an often needed use-case is to display labels with a key and name attribute. Something like "AW3 - Active Workgroup 3"

Usage of this feature as follows:

```php
$options = array(
    'label_generator' => function($targetEntity) {
        return $targetEntity->getKey() . ' - ' . $targetEntity->getName();
    }
)
```