Move nose configuration out of base settings

We only install nose in the requirements for testing, so don't add nose to
INSTALLED_APPS in the base settings; do it in test.py instead. The
settings/test.py file is new; it mostly contains things that were formerly in
travis.py (which now imports from test.py), plus the nose settings.

Travis should run as before. To run tests manually, do

  python manage.py test --settings=settings.test