Extract slug generator to separate method

The main reason for changing this is that all standard php methods of transliaterationg are unstable (like iconv) or unusable for russian (Transliterator class). So that I managed to extract slug generation to separate method which can be redefined without need to copypaste.

In addition, I created Utils class with static function which applies traditional russian way of transliteration.

This look kinda weird. Inception function in a method.

If you need to pass parameter, try `call_user_func_array`

Also it would be nice to keep inner compatibility of this package. Checkout this [localeCallable](https://github.com/KnpLabs/DoctrineBehaviors/blob/master/src/Knp/DoctrineBehaviors/ORM/Translatable/TranslatableListener.php#L186-L191) in Translatable.