Add --group-by parameter to specify a unique tag value shared by a raid group

...of a raid group share (e.g. timestamp from the time before the snapshots were taken)

Example:

Snapshot 1 tags:
db=001
volume=001
timestamp=1375833666

Snapshot 2 tags:
db=001
volume=002
timestamp=1375833666

Snapshot 3 tags:
db=001
volume=003
timestamp=1375833666

--keep-hourly=24 --keep-daily=7 --keep-weekly=4 --keep-monthly=1 --by-tags db=001 --group_by=timestamp --dry-run