Switch to nose and get base (non-selenium, non-country-specific) tests passing

This is the first pull request in a set of work aimed at getting the tests in
pombola into better shape; it gets all the base tests passing under Nose,
but there are still the following issues to be addressed:

* https://github.com/mysociety/pombola/issues/1181

Lots of the changes here are required because Nose loads every module
to see if it contains tests to run, and some modules in Pombola will only
import without error with database objects to specific countries, or with
a particular COUNTRY_APP set. There are also some fixes for old code
that is only for reference, and which no longer imports without error.

The most significant commit may be a5f12b7f3596f2b3588c329548f34
which introduces per-country (and no-country) settings modules.