Add support for multiple translator domains

Add support additional translator domains other than the default `messages` domain (http://symfony.com/doc/current/components/translation/introduction.html#using-message-domains).

### Example:
Let the current `en` translation file live at `app/translation/en.yml`.

[`messages` is the default domain](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Translation/Translator.php#L96), ergo `app/translation/en-messages.yml` is the same as `app/translation/en.yml`

If one wanted to create [validation translations](http://symfony.com/doc/current/book/translation.html#book-translation-constraint-messages) under the `validators` domain, simply create `app/translation/en-validators.yml`