Feature/redirect to https

There's no way of configuring the ELB to redirect; it just terminates HTTPS there and forwards everything to port 80. So I tweaked the ELB listener to forward 80 -> 80 and 443 -> 443 (but HTTP, not HTTPS). That way, nginx can reply to anything coming in on 80 with a 301. ( @dlapiduz, take note.)
(In theory, it's possible to get nginx doing this with a single server listener since ELB sends the origin port details through, but this requires use of `if` which has [all kinds of nasties](http://wiki.nginx.org/IfIsEvil).)

To make the site happy on HTTPS, a few asset references needed changing. Placeholder images have been downloaded for the moment because [placehold.it](http://placehold.it) doesn't do HTTPS.
Same note here about the `preload` attribute for HSTS.