Fixed specs and slug for multi-language

Hi,
in the first commit I've fixed the specs with the new Globalize gem.
In the second commit I've added the possibility to create a translated object with all translations in one single step. This is useful to integrate with ActiveAdmin (though it's not tested directly with that).
I've tested the new feature. Let me know if you need any explaination. Cheers!
So the problem is that my ```super_set_slug``` is slightly different from the super, in particular in the last line in which we call ```translation.slug = slug```. Do you have any idea on how to do that in a better way?