A maybe better panel's markup

Hi,

A two points introduction:
<ul>
<li>Thanks for this wp theme! I started to build a _s + bootstrap theme too, but I stopped to work on it after I saw _tk!</li>
<li>This is my very first pull request on Github. So I apologize in advance if I made some mistake (and for my bad  english too).</li>
</ul>

This is why I'm making this pull request:
<ul>
<li>I saw a small error on a copied-and-paste comment on functions.php</li>
<li>I noticed that bootstrap doesn't style panels correct if they haven't a .panel-default (or .panel-primary etc.). For example they have transparent border color.</li>
<li>It's missing .panel-heading and .panel-body classes too.</li>
<li>At last, if a comment has the max depth, it shows no reply button, so we get an empty footer.</li>
</ul>