Use X-Forwarded-Proto to determine is_secure

Use the X-Forwarded-Proto header to determine whether the request is
secure or not. This allows Stagecraft to correctly generate URLs to
itself.

https://docs.djangoproject.com/en/1.5/ref/settings/#secure-proxy-ssl-header
Merge pull request #129 from alphagov/use-forwarded-proto

Use X-Forwarded-Proto to determine is_secure