Modified the way EMAIL_BACKEND is specified to make it consistent with the new...

Modified the way EMAIL_BACKEND is specified to make it consistent with the new "use the class name" policy for backends.

This is a BACKWARDS-INCOMPATIBLE CHANGE for anyone using a manually
specified EMAIL_BACKEND setting. If you have manually specified
EMAIL_BACKEND, you will need to append ".EmailBackend" to your
existing EMAIL_BACKEND setting. See the django-dev mailing list for
details.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12084 bcc190cf-cafb-0310-a4f2-bffc1f526a37