Mark gettext functions used as translator attributes

Addresses https://github.com/Yelp/yelp_cheetah/issues/162

Included some acceptance test. Someone more familiar with the organization of this test suite should be able to confirm if this was indeed the right place to have this test.

cc @asottile @bukzor 
So do I. However, I value more consistency than any particular coding style. Since I am not an official maintainer, I am trying to stick to the existing conventions.