Fixed #16099 -- Enabled threading for the runserver management command and added a --nothreading option to disable it if needed. This should help Google Chrome users because it opens more than one connection speculatively.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16427 bcc190cf-cafb-0310-a4f2-bffc1f526a37