Added contrib apps to ignore list when running makemessages in the Djagno...

Added contrib apps to ignore list when running makemessages in the Djagno source tree and added a --no-obsolete option in preparation of moving app translations out of core.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15258 bcc190cf-cafb-0310-a4f2-bffc1f526a37