Added support for downloading backup as a tar archive…

[oll3]

Added support for downloading a (gzip compressed) tar archive. Mainly did it for my own use as I needed to restore a backup bigger than 4 GB (which is too big for normal zip to handle), but
though I might as well publish it in case it's of any use to someone else.

Adds dependency of libarchive.

[uroni]

Thanks! Adding the dependency to the Windows server is some work, which is why I used MiniZ initially (and I thought ZIP64 support was around the corner there...).
UrBackup 2.0 (dev branch) sets the last modified time etc. in the ZIP correctly. For tar.gz one can also return symbolic links, special files etc. correctly. But it also can directly restore, so I think I'll wait and see how many people hit the 4GB limit there.

[oll3]

No problem! Understand it as libarchive should work fine under windows, and think zip64 support is on its way. But I can see that there is still work that needs to be done (on your behalf) to get it up and running on all platforms, so I guess it seems reasonable to wait.