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 
Yeah, It wasn't clear to me we needed to support cases where the dotted based name includes callable e.g. `$foo.bar().baz().womp().herp().derp().gettext('...')`

I was working under the premise that this isn't a good practice for using gettext, but of course, this knowledge shouldn't be embedded in the compiler.