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
Ref #652 - Add installation/config info to docs TOC
Remove disabled post nav links
Add section about grunt to README
Simplify alerts
Update editor-style.css

- Bring in latest Bootstrap changes

- Add instructions on how to update the file
Update danish lang files
Update CHANGELOG with changes since 6.3.0
Comments clean up, consistent formatting