dar-backup version 1.3.0

Switched to semantic versioning, no dramatic changes

New functionality:

• a cleanup script to remove old (configurable) DIFF & INC archives. FULL archives are never touched
• cleanup systemd timer and service provided (not automatically installed)
• par2 integration refactored, logging improved, docs improved
• install instructions simplified
• alerts for upcoming FULL and DIFF backups sent
• minor improvements to logging in general