Use Bootstrap media object for listing comments

http://twitter.github.com/bootstrap/components.html#media

- Introduce lib/comments.php: contains Roots_Walker_Comment

- Update templates/comments.php to use the new walker, general
clean up

![new-comments](https://f.cloud.github.com/assets/115911/147823/87c134ce-74d8-11e2-8dc9-7e070bf5bff6.png)