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!
probably the real issue here is that the original `set_slug` is doing two things: generating the url and setting it.