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 
heh, then by that it should be `gettextScannables`

for new code I've been using snake case and converting-on-write the rest of the code.