raise a meaningfull error on ascii encoding

Filed https://bugzilla.mozilla.org/show_bug.cgi?id=925968 to fix up m2secret, which isn't going to be easy.

Right now django-aesfield only works with ascii, this just makes it explicit and allows to upgrade to Django 1.5, which casts all input as utf8. The only fields we use this on are uuid like fields we generate.