Added CURL progress function callback when downloading/uploading.

[racklin]

Add a progress function when S3 downloading / uploading

S3::setProgressFunction('progress');

function progress($resource,$download_size, $downloaded, $upload_size, $uploaded)
{
    if($download_size > 0)
         echo $downloaded / $download_size  * 100;
    ob_flush();
    flush();
    sleep(1); // just to see effect
}

[tpyo]

Again, sorry for the late response @racklin. Could you please make some code-styling tweaks? Thanks!