Support for lazy translations

One more fix... :)

Turns out `ugettext_lazy` proxies weren't correctly supported for labels, and since we (@andersinno) tend to work on things that require localization, support for that is needed.

Luckily, turns out it's a simple fix -- just call `force_text()` in `__str__`.

FWIW, this is probably the "correct" fix for 05af4face38.