Add target parameter for arbitrary URL schemes

This allows backups to SSH servers or every other URL scheme supported
by duplicity. The old cloud, bucket and folder parameters are deprecated
but still supported.

I opted to keep the existing cloud, bucket and folder parameters but to mark them as deprecated. Existing manifests should continue to work as before, at least all test still pass unmodified. If you prefer changes (droping the existing parameters right away or keeping them as the standard method), I'm open for discussion.

Thanks for considering this branch
Gaudenz