Add a healthcheck route

This adds a healthcheck route for the app which returns a 200 OK. This
is used by the Nagios alert to check if the app is responding in a 
timely fashion.
Merge pull request #1 from alphagov/add-healthcheck

Add a healthcheck route
Merge commit '41c92bace71f24809e4ad23231a9f60869b39a8c' into HEAD