Changes in version 0.4.1:

Bugs fixed:
- Fixed a possible deadlock in the archiver which could cause `rustic backup` to hang.
- Piping output no longer panices (this allows e.g. to pipe into `head`).
- Fixed progress bar showing 0B/s instead of real rate.

New features:
- backup: Errors reading the parent now print a warning instead of being silently ignored.
- forget: Allow to keep quarter- and half-yearly.
- Improved the error handling for some situations.
