replace favicon with several sizes version and only use the tortoises head

source/layouts/layout.erb

@@ -16,6 +16,9 @@
         <% end %>
     </title>
     <link rel="shortcut icon" href="img/favicon.ico" type="image/x-icon" />
+    <link rel="icon" href="img/favicon-16x16.png" type="image/png" sizes="16x16" />
+    <link rel="icon" href="img/favicon-48x48.png" type="image/png" sizes="48x48" />
+    <link rel="icon" href="img/favicon-32x32.png" type="image/png" sizes="32x32" />
 
     <%= stylesheet_link_tag "bootstrap" %>
     <%= stylesheet_link_tag "base" %>