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

lib/comments.php

@@ -0,0 +1,64 @@
+<?php
+
+function roots_get_avatar($avatar) {
+  $avatar = str_replace("class='avatar", "class='avatar pull-left media-object", $avatar);
+  return $avatar;
+}
+add_filter('get_avatar', 'roots_get_avatar');
+
+/**
+ * Use Bootstrap's media object for listing comments
+ *
+ * @link http://twitter.github.com/bootstrap/components.html#media
+ */
+class Roots_Walker_Comment extends Walker_Comment {
+  function start_lvl(&$output, $depth = 0, $args = array()) {
+    $GLOBALS['comment_depth'] = $depth + 1; ?>
+    <ul <?php comment_class('media unstyled comment-' . get_comment_ID()); ?>>
+    <?php
+  }
+
+  function end_lvl(&$output, $depth = 0, $args = array()) {
+    $GLOBALS['comment_depth'] = $depth + 1;
+    echo '</ul>';
+  }
+
+  function start_el(&$output, $comment, $depth, $args, $id = 0) {
+    $depth++;
+    $GLOBALS['comment_depth'] = $depth;
+    $GLOBALS['comment'] = $comment;
+
+    if (!empty($args['callback'])) {
+      call_user_func($args['callback'], $comment, $args, $depth);
+      return;
+    }
+
+    extract($args, EXTR_SKIP); ?>
+
+  <li <?php comment_class('media comment-' . get_comment_ID()); ?>>
+    <?php echo get_avatar($comment, $size = '64'); ?>
+    <div class="media-body">
+      <h4 class="media-heading"><?php echo get_comment_author_link(); ?></h4>
+      <time datetime="<?php echo comment_date('c'); ?>"><a href="<?php echo htmlspecialchars(get_comment_link($comment->comment_ID)); ?>"><?php printf(__('%1$s', 'roots'), get_comment_date(),  get_comment_time()); ?></a></time>
+      <?php edit_comment_link(__('(Edit)', 'roots'), '', ''); ?>
+
+      <?php if ($comment->comment_approved == '0') : ?>
+        <div class="alert alert-block fade in">
+          <a class="close" data-dismiss="alert">&times;</a>
+          <p><?php _e('Your comment is awaiting moderation.', 'roots'); ?></p>
+        </div>
+      <?php endif; ?>
+
+      <?php comment_text(); ?>
+      <?php comment_reply_link(array_merge($args, array('depth' => $depth, 'max_depth' => $args['max_depth']))); ?>
+  <?php
+  }
+
+  function end_el(&$output, $comment, $depth = 0, $args = array()) {
+    if (!empty($args['end-callback'])) {
+      call_user_func($args['end-callback'], $comment, $args, $depth);
+      return;
+    }
+    echo "</li>\n";
+  }
+}