Use Transifex for translation.

More and more django packages make use of [Transifex](http://transifex.com), a collaborative translation platform that's free for open-source usage. Wanting to improve the German translation of `django-allauth` I tried to kill to birds with one stone and set up a [project page over at Transifex](https://www.transifex.com/projects/p/django-allauth) and imported the current resource files. Feel free to visit the site, add new languages and improve existing ones.