http://stackoverflow.com/questions/15833546/overwrrite-code-igniter-common-php?noredirect=1#comment22527358_15833546

gistfile1.php

<?php

class MY_Controller extends CI_Controller {

    public function __construct() {
        parent::__construct();
        require_once APPPATH. '/libraries/MtHaml/Autoloader.php'; // or whereever you put that file
        \MtHaml\Autoloader::register();
        $this->haml = new MtHaml\Environment('twig');
    }

}