[WIP] Translator: keeps domain in untranslated message, vol. 2

So... I tried to implement https://github.com/Kdyby/Translation/pull/48#issuecomment-51616141

It almost works, with one problem... because of https://github.com/symfony/Translation/blob/master/Translator.php#L205 and similar line in transChoice, my NULL value from MessageCatalogue is always returned as empty string. And I don't think it's safe to assume, that returned empty string means untranslated. 

I have two possible solutions for this, but cannot decide which one to choose:
- return some special character/string from catalogue instead of null
- or just copy trans/transChoice methods from Symfony to Kdyby and make neccessary changes

What do you think? Or do you have some better idea?