FORMATING: remove tailing "/" from $BACKUP_SCRIPTS_DIR
 - caused a double-slash in "run-parts"-call
IMPROVE: add test for required external commands
IMPROVE: reduce system-load
- by "nice"-ing execution of tar
IMPROVE: use config-file for specifying SVN-repos
 - configuration-parameter: SUBVERSION_REPOS
 - backup-files will be prefixed by "subversion_"
IMPROVE: reduce system-load
 - by "nice"-ing execution of svnadmin & bzip2
 - as commit "fd1a621a276a2650e168f6ce8a940c2ce799f1ff"