Possibility to call methods on entities, possibility to generate unique id

Hi. This PR allow you to call methods on entities. Also it allow you to generate unique id even if your id is not autogenerated.

So you can call methods and generate id like this:
```php
$generator = Factory::create();
$populator = new Populator($generator, $em);
$populator->addEntity('Something', 1, array(), array(
    'setName' => array($generator->name)
), true);
$populator->execute();
```