Add support for "snapshot"

https://bitbucket.org/gordonmessmer/dragonsdawn-snapshot

Similar to the existing LVM snapshot support with some notable improvements.  This works both locally and for ssh backups.  This supports syncing data to disk (such as making SQL database files consistent) before backup.  This supports filesystems other than ext4 on LVM (such as btrfs, Gluster, and NetApp NFS).  This is also aware of and will avoid snapshots that can potentially destroy a filesystem (such as making a snapshot of an ext filesystem with an external journal).