Improved PhraseTranslator

In order to avoid problems down the road I improved the implementation of the Phrase Translator, this is what you should recommend to your users as it follows better OO practices.

* implemented support for both `trans` and `transChoice`
* removed duplicated code from BaseTranslator and made the translator act only on the ID as needed, reducing the chances of changes upstream not showing up locally.

I'm also working on a proper Symfony Bundle to implement all you describe, once its ready i'll share in case its interesting for you.