Add Basic-Auth - callers authenticate with a GitHub access token
a bit more doc...
Tweaks to app.json

Note we need to specify the repositoy template url for the 'Deploy To
Heroku' button:

https://github.com/guardian/gu-who/pull/34
More doc
More Readme tweaks
Add code to work out what folders a pull-request touched

So at some point, prout config will only fire for a PR if the PR
made changes that occured at or below the folder containg the
.prout.json