Updated tests for backuprepository and cron.
Fixed wrong order of the global ranges tuple.
Added unmount() method to Device.
Added two methods.

One for starting scheduling, which was done in the constructor so far. The
other one checks all backups in all backup repositories manually, which is
necessary as not all backups might expire or become necessary while the program
is running.
Finished raw structure of autobackup.py.

But beware, this is not tested at all.