Allow to track upload and download progress

[schwa]

This issue was brought up by Ian Cook, see http://codespeak.net/pipermail/ftputil/2006q3/000124.html .

When an upload or download is started, there's no way to follow the progress of the transfer (other than stat'ing the target file). The ftputil code should be extended to follow the transfer's progress, possibly with a callback function/method.