Escape bug fix and few tweaks

If the Jenkins Task name would be ``Backup /var/lib/jenkins`` then previously ``rm -rf $ARC_DIR`` would kick off ``rm -rf /var/lib/jenkins/workspace/Backup /var/lib/jenkins``. Accordingly I've added escaping everywhere just to be safe.

Additionally I've changed archive paths so that they would respect given target in respect to workspace directory. For example ``./jenkins-backup.sh /var/lib/jenkins ./backup/20141127160526.tar.gz``