Fixed #15042 -- Ensured that email addresses without a domain can still be mail recipients. Patch also improves the IDN handling introduced by r15006, and refactors the test suite to ensure even feature coverage. Thanks to net147 for the report, and to Łukasz Rekucki for the awesome patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15211 bcc190cf-cafb-0310-a4f2-bffc1f526a37