Place in steamapps\common\arma 2 operation arrowhead\@DayZ\addons and run. Requires php/winrar on PATH.

DayZ_update.php

<?php
$url = 'http://cdn.armafiles.info/';
$filelist = trim(file_get_contents($url.'md5checksums.txt'));
$lines = explode("\n", $filelist);

$oldfiles = @unserialize(file_get_contents('files'));
foreach ($lines as $line) {
    list($md5, $file) = preg_split('/\s/', $line,-1,PREG_SPLIT_NO_EMPTY);
    $files[$md5] = $file;

    echo "verifying $file ($md5)\n";
    if(!in_array($md5, array_keys($oldfiles))){ //changed file
        echo "Updating $file...\n";
        file_put_contents($file, file_get_contents($url.$file));
        echo "Extracting $file...\n";
        system("rar e $file -o+");
        unlink("$file");
    }
}
echo "Done.";
file_put_contents('files', serialize($files));
?>