#16 incremental backups

[Wandermind]

Backing up even medium disks can be quite consuming, especially when done regularly. This could be perhaps improved by considering incremental backups (if it's worth the work required to implement it). The idea:
- Create librsync (AKA rdiff) signature for the last full backup (or maybe some other selected by the user). This would be done only once for each full backup, probably at the time of its first incremental backup.
- When processing data from disks, use the signature of the selected full backup and produce and save just a delta instead of all the data.
- When recovering, use rdiff patch algorithm to merge the selected incremental backup delta and its corresponding full backup.
This would save time on compressing and saving the data, especially for slow compression algorithms like bzip2 or lzma and/or slow storage.
It would have to be done on uncompressed data to work properly, so compressed backups would have to decompressed when generating the signature.