Add rsync backups with hardlinks for space savings.

This is a modification to the backup scheme that I've been using on my server for some time.  My .zip files, while not huge, were piling up and taking quite a lot of disk space, and it occurred to me that 90%+ of the data was probably unchanged each time, which is a big waste.  Instead of using zip, this method just uses rsync to copy the directory structure over, with hardlinks to files that haven't changed since the last backup.  This makes each backup larger than the .zip file if hardlinks are ignored, but the actual disk usage is vastly lower.

I made my change originally as a hack to 0.8.15, and it's been working for me for quite a while.  When I decided to send you my changes, I forked the latest master and saw your addition of rdiff-backup.  I still think my method has value, because it's compatible with WorldEdit snapshot restores, which I use a lot.  For this pull request I've attempted to add it as an optional backup method the same way you did for rdiff-backup, but I haven't tested this version much.