Added the fallback path through multipart locales (e.g. 'de_DE' > 'de' > 'en') parts before fallback is used instead of just falling back ('fr' > 'en')

The symfony2 way of falling back through locales is to walk through the eventually parts 
e.g. 'de_DE' > 'de' > 'en' (fallback). The js translation falls directly back to the fallback and doesn't checks parts of the given locale 'de_DE' > 'en'.

I implemented the symfony2 behaviour in the 'Translator.get_message' and would love to see it ending up in the bundle. Let me know, if you have any issues, about the code or the tests (btw: should i update the documentation anywhere?)!