FTP support, less (or different) bugs, updated documentation.

  * Add FTP upload, requires lftp.
  * Correct a few bugs:
    - Don't use -a option with tar, since older versions don't understand it.
  * Small improvements
    - Use functions to set the final status (SUCCESS, WARNING, ERROR, CRITICAL)
  * Update documentation:
    - Document the FTP part.
    - Add a section about requirements.
    - Add a warning about REPODIR being on a backed up device.
    - Remove examples from the script itself, everything is in README.
    - Some cosmetic changes.
    - Renamed briochetab to briochetab.example.