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 was hoping by adding the test case I was talking about you'd realize that this line of code needs to be this much simpler version:

```
if any(name_chunk[0] in self.setting('gettextTokens') for name_chunk in name_chunks):
    self.addGetTextVar(nameChunks, lineCol)
```