Add middleware to inject expected ssl request header.

Azure does not provide the request headers expected by Express
for secure requests. Add a middleware to inject the necessary header.
See
http://stackoverflow.com/questions/15084511/how-to-detect-https-redirection-on-azure-websites
for details.
Merge pull request #20 from alphagov/redirect_to_ssl_done_page

Add middleware to inject expected ssl request header.