Browsing recent posts

See [original user story](https://www.pivotaltracker.com/story/show/47578825).

To do:

* [x] Include `created_at` as publication date in the markup somehow.
* [x] Use `article` elements as post wrappers
* [x] Use `time` elements with `pubdate` attribute to mark up publication date.
* [x] Mark links to single posts with `rel="bookmark"`
* [x] Highlight most recent post somehow