Use an identifier for Disqus

Disqus creates a new thread for comment from a different URL. This means that the page visited from HTTP have a different thread than the one on HTTPS. Work around this using `page.id` as `disqus_identifier`. `page.id` is guaranteed to be unique by Jekyll:

> ## Page Variables
>
> VARIABLE | DESCRIPTION
> ----|----
> `page.id` | An identifier **unique** to the Post (useful in RSS feeds). e.g. `/2008/12/14/my-post`

From http://jekyllrb.com/docs/variables/