Reference language selector

A couple of months ago, we added a reference language selector in django-rosetta and it's been sitting in our private fork of it since then. We thought you might like it, so we've cleaned it up and pushed it here.

The feature adds a Reference language selector to the translation interface. This way, instead of having only the option to show the msgid to translate, you can also have the translated version (if you already have other translations) of the msgid. That's useful if you have translators that don't speak your base language.