Fast backup using BTRFS and rsync

if BTRFS is available one should be able to use its snapshot feature to realize the incremental backup.
* On creating the backup task a BTRFS subvolume is created on the destination volume
* On updating the backup a snapshot of this subvolume is created and the files are then transferred using rsync.

this of course only works for local drives (HDD, USB) where the file system type can be determined. Once BTRFS supports transparent encryption this can be added.

In the scope of this blueprint is also factoring out the duplicity code to allow different backends, like this BTRFS specific one.