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
Merge pull request #683 from retlehs/bs_media_object_comments

Use Bootstrap media object for listing comments
Ref #652 - Add installation/config info to docs TOC
Remove disabled post nav links
Add section about grunt to README
Simplify alerts
root simplify alerts, bs media object for comments