Add --snapshot-prefix and free choice for --snapshot-folder

With this changes it is possible to use a directory which is not
located within the subvolume as a location for the snapshots.
Additionally the names of the snapshots can be prefixed. This allows to
use one target btrfs pool with many sources.
Example:
#btrfs-backup.py --snapshot-folder /mnt/btrPool --snapshot-prefix vault
/mnt/btrPool/vault/ /mnt/btrBackup