Enhanced rsync handler.

- Added support for:
  - Remote destinations
  - Long rotation format similar to maildir handler
  - Batch files through --read-batch and --write-batch
  - Custom file list using --files-from
  - SSH persistent connection using ControlMaster
  - The rsync:// protocol
- Metadata folder for each backup folder
- General refactoring
- Code cleanup

Not all options were tested and it might contain bugs.
Tests, comments and patches are welcome. :)