client-side implementation of the rsync algorithm

 zsync is a file transfer program to download files from
 remote web servers. If a previous version of a file is available
 locally, zsync will only download changed parts and hereby
 minimise the download volume. The algorithm is the same as used
 by rsync(1), but zsync does not require any server software
 (apart from a web server), nor does it need shell access.
 Instead, it uses a control file (.zsync file) that describes the
 file to be downloaded, which it uses to determine the blocks to
 fetch. This file is created once on the server (and not for each
 request) and sits next to actual file to download