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 had the same doubt... there are already instance variables in this same class using the `List` suffix (~50% of the existing lists). This could use a good cleanup but I am not planning on doing so in this branch.

For now, lets break the pattern and stop using the suffix then ;)