Add UUID of the snapshot taken to the backup archive
- Added option (--delete-backup-snapshot) to delete the snapshot taken
  during backup after restore
- Read UUID of latest backup snapshot taken from the backup archive
- Added show_usage function
- Small clean ups
Added documentation for new options
Merge branch 'master' of https://github.com/Rackster/vzpbackup into Rackster-master

Conflicts:
	vzpbackup.sh
	vzprestore.sh
Added pz/pigz option to compression