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 
When I meant to split these up, each 3 line chunk should be its own test method since they're all independent.

I also don't believe these fail even when `addGetTextVar` is `pass` so you're not really testing the feature