Use `usegmt` flag in formatdate

Slightly cleaner and faster than string manipulation.
 
This flag has been available since Python 2.4:
http://docs.python.org/2/library/email.util.html#email.utils.formatdate
Merge pull request #1447 from evansd/patch-1

Use `usegmt` flag in formatdate
Fixed #20868 -- Added an email to django-announce as a security step.

Thanks garrison for the report.
Fixed #20828 -- Allowed @permission_required to take a list of permissions

Thanks Giggaflop for the suggestion.
Test that django.views.static.serve() generates 404 status codes.

Also, change tests to be based on SimpleTestCase.
Expand testing of Test LiveServerTestCase static files serving.
clarified misleading wording about squashing commits
Merge pull request #1464 from evildmp/squashing_commits

Clarified misleading wording about squashing commits
Fixed #20890 -- Added missing import in class-based view docs.

Thanks André Augusto.
Fixed #13518 -- Added FILE_UPLOAD_DIRECTORY_PERMISSIONS setting

This setting does for new directories what FILE_UPLOAD_PERMISSIONS
does for new files.

Thanks jacob@ for the suggestion.
Fixed #20883 -- Made multi-table inheritance look for explicit parent_link in abstract parents.