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

functions.php

@@ -12,6 +12,7 @@ require_once locate_template('/lib/config.php');          // Configuration
 require_once locate_template('/lib/activation.php');      // Theme activation
 require_once locate_template('/lib/cleanup.php');         // Cleanup
 require_once locate_template('/lib/nav.php');             // Custom nav modifications
+require_once locate_template('/lib/comments.php');        // Custom comments modifications
 require_once locate_template('/lib/rewrites.php');        // URL rewriting for assets
 require_once locate_template('/lib/htaccess.php');        // HTML5 Boilerplate .htaccess
 require_once locate_template('/lib/widgets.php');         // Sidebars and widgets