Object: provide ::getClassName()

This helper is useful for working with class names. Triggering autoloading is not a problem here, I use the class anyways.

```php
$articles = $entityManager->getRepository(App\Article::getClassName());
$article = $articles->find(1);
```

Related:

* http://marc.info/?l=php-internals&m=123257207431054&w=2
* https://wiki.php.net/rfc/class_name_scalars

I know it's in 5.5, but let's be reasonable, how long will it take to get 5.5 supported to be able to use it in global opensource projects? Five years at least? 

Its 2013, 5.2 is dead and there are still applications dependent on it, and not even 5.4 is widenly supported.