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 
I'll going to try to take a look at this today https://github.com/Yelp/yelp_cheetah/blob/master/Cheetah/legacy_compiler.py#L651-L652

If it turns out to be more than 1-day change, I will have to punt it to a followup branch since I don't have the bandwidth to do more than that right now.