Php Laravel 4辅助类未自动加载

我正在尝试使用Laravel 4中位于

app/helpers

文件夹中的helper类，但无论我做什么，Laravel都会给我错误

找不到类

助手基类

Helper.php
<?php
/* 
    Base Helper class!
*/

class Helpers {

}

Helper.php
发现了问题

在Gravatar.php中，

<? 
class Gravatar extends Helpers {
 /**
     * Get either a Gravatar URL or complete image tag for a specified email address.
     *
     * @param string $email The email address
     * @param string $s Size in pixels, defaults to 80px [ 1 - 2048 ]
     * @param string $d Default imageset to use [ 404 | mm | identicon | monsterid | wavatar ]
     * @param string $r Maximum rating (inclusive) [ g | pg | r | x ]
     * @param boole $img True to return a complete IMG tag False for just the URL
     * @param array $atts Optional, additional key/value attributes to include in the IMG tag
     * @return String containing either just a URL or a complete image tag
     * @source http://gravatar.com/site/implement/images/php/
     */
    public static function get_gravatar( $email, $s = 80, $d = 'mm', $r = 'g', $img = false, $atts = array() ) {
        $url = 'http://www.gravatar.com/avatar/';
        $url .= md5( strtolower( trim( $email ) ) );
        $url .= "?s=$s&d=$d&r=$r";
        if ( $img ) {
            $url = '<img src="' . $url . '"';
            foreach ( $atts as $key => $val )
                $url .= ' ' . $key . '="' . $val . '"';
            $url .= ' />';
        }
        return $url;
    }
}

<img src="{{ Gravatar::get_gravatar($thread['user']['email'], 50) }}">