Remove unnecessary import.
Fix wrong loop, only a pass in a loop makes zero sense dude.
Extend debug messages.
Add general outline of extended backup info.
Implement new folder structure.

Look into doc/ideas/extended-backup-info.txt for further details. Some
changes that are entangled into that:

- The configuration options for overlapping backups was removed.
  Symlinking was the objectively superior method anyway, so the support
  for hardlinking and only creating a single backup was dropped.
- A lot of functionality, mostly about parsing, creating and removing
  backups, was moved from rbackupd/__init__.py to
  rbackupd/repository.py.
- All in all, the code is a bit more "elegant" now. No more classes that
  only hold data, but proper reponsibility for their scope. Some new
  classes were added to achieve this.
Fix wrong name assignment to new backups.

Newly created non-symlink backups would be assigned the name of the
repository instead of their proper folder name.
Enable some kind of filesystem parsing.

The repository folder is now parsed ONCE at the beginning. When backups
are created or deleted, they are added to the list without accessing the
filesystem.
Exit when keep value is zero or less.

Negative values were not supported before, now zero is invalid, too. The
reasons are:
1) zero makes no sense anyway, as it would create a backup and remove it
   immediately every time.
2) It would break the "latest" symlink, as the newly created backup
   would be the target of the "latest" symlink and be removed after
   that.
Add debug messages.
Merge branch 'extended-info' into develop