Simple integration tests

This provides a very basic framework for running simple integration tests.

Tests can be run either locally in Firefox or remotely against Saucelabs.

You can also point the tests at any already running environment e.g. local, CODE or PROD (if running remotely then the firewall will prevent you using Saucelabs to access localhost)

Also includes a simple set of [***integration test principles***](https://github.com/guardian/frontend/blob/simple-integration-tests/integrated-tests/README.md). I'm particularly keen for comment on that.
Same precedence as Guardian config library