[WIP] Filedriver refactor

Refactor the FileDriver and YamlDriver. The Yaml mapping for translation locales is updated for consistency. A DoctrineAdapter is added to build a metadata driver based on the doctrine driverchain. Advantages:

* Better testability for drivers
* Ability to keep Doctrine and Translatable mappings separate
* Easier configuration through DoctrineAdapter
* Better performance for the Yaml driver. The Doctrine driver chain is parsed only once, not for every class.