Allow <img alt> by default

Includes a regression test and a bit of test cleanup.  Unfortunately the ``assertHTMLEqual`` test case method isn't available in Django 1.3, so the test doesn't run there (see comments in the diff).  I'm guessing that this doesn't really matter, though, since the import of ``from django.utils.unittest import skipIf`` would have broken the tests in Django 1.3 anyway.