Multibyte support

Dear author,

I want this plugin to support multibyte languages like Japanese, Chinese, and so on.
I think it’s easy. A little bit of change on top-core.php after L.402.
Set $lenFunc/$subFunc and use them as strlen/substr function like this:

//Shrink a tweet and accompanying URL down to 140 chars.
function set_tweet_length($message, $url, $twitter_hashtags="", $hashtag_length=0) {

	$lenFunc = function_exists('mb_strlen') ? 'mb_strlen' : 'strlen';
	$subFunc = function_exists('mb_substr') ? 'mb_substr' : 'substr';
    $tags = $twitter_hashtags;
    $message_length = $lenFunc($message);
    $url_length = $lenFunc($url);
    //$cur_length = $lenFunc($tags);
    if ($hashtag_length == 0)
        $hashtag_length = $lenFunc($tags);

    if ($twitter_hashtags != "") {
        if ($lenFunc($tags) > $hashtag_length) {
            $tags = $subFunc($tags, 0, $hashtag_length);
            $tags = $subFunc($tags, 0, strrpos($tags, ' '));
        }
        $hashtag_length = $lenFunc($tags);
    }

    if ($message_length + $url_length + $hashtag_length > 140) {

        $shorten_message_to = 140 - $url_length - $hashtag_length;
        $shorten_message_to = $shorten_message_to - 4;
//$message = $message." ";
        if ($lenFunc($message) > $shorten_message_to) {
            $message = $subFunc($message, 0, $shorten_message_to);
            $message = $subFunc($message, 0, strrpos($message, ' '));
        }
        $message = $message . "...";
    }
    return $message . " " . $url . " " . $tags;
}

This is important for mutlibyte language users like me, as this problem can’t be fixed with wp filters.
Thank you for your consideration (and patience with my bad English).

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