Add a feature for Bouncer

This makes a request directly to Bouncer rather than visiting the
URL and checking that it redirects (as the Redirector features do)
because we will be migrating sites from Redirector to Bouncer over
several months, so there would be no guarantee that that request
would be being sent to Bouncer by the CDN rather than Redirector.
Clarify Bouncer request step

This does make a request directly to Bouncer rather than visiting
the page and the request going through the CDN and then to Bouncer,
so this description makes that clearer.
Merge pull request #54 from alphagov/add_bouncer_feature

Add a feature for Bouncer