CodeIgniter | Obtener automáticamente la base_url

Sólo debes reemplazar

$config['base_url'] = "http://misitio.com";

con las siguientes lineas:

$config['base_url'] = ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == "on") ? "https" : "http");
$config['base_url'] .= "://".$_SERVER['HTTP_HOST'];
$config['base_url'] .= str_replace(basename($_SERVER['SCRIPT_NAME']),"",$_SERVER['SCRIPT_NAME']);