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.
if you're already using HSTS and for all subdomains, you could also add `; preload` to the end. That does nothing but let you automatically [get added to Chrome's HSTS list](https://hstspreload.appspot.com/) (indicates the web owner has consented to the preload).