And an end-to-end Selenium-based test

This adds a test which:
- starts Chrome, with the app installed and the browser's proxy settings pointed to localhost:9999
- fetches a few pages and verifies they load

I added m.guardian.co.uk because right now it *doesn't* load via the proxy. No idea why!

Caveats:
- requires the selenium server to be running locally (there are a few grunt plugins which purport to do this but I couldn't get them to work)
- no Travis/Sauce Labs connection yet...uploading the extension will take a little more work (it seems to be possible but let's take this in stages, there's other things to do too!)