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