directory structure change to work with glide

glide has its own requirements. My previous workaround caused me some
code checkin errors. Need to fix this.

weed/command/backup.go

@@ -0,0 +1,90 @@
+package command
+
+import (
+	"fmt"
+
+	"github.com/chrislusf/seaweedfs/weed/operation"
+	"github.com/chrislusf/seaweedfs/weed/storage"
+)
+
+var (
+	s BackupOptions
+)
+
+type BackupOptions struct {
+	master     *string
+	collection *string
+	dir        *string
+	volumeId   *int
+}
+
+func init() {
+	cmdBackup.Run = runBackup // break init cycle
+	s.master = cmdBackup.Flag.String("server", "localhost:9333", "SeaweedFS master location")
+	s.collection = cmdBackup.Flag.String("collection", "", "collection name")
+	s.dir = cmdBackup.Flag.String("dir", ".", "directory to store volume data files")
+	s.volumeId = cmdBackup.Flag.Int("volumeId", -1, "a volume id. The volume .dat and .idx files should already exist in the dir.")
+}
+
+var cmdBackup = &Command{
+	UsageLine: "backup -dir=. -volumeId=234 -server=localhost:9333",
+	Short:     "incrementally backup a volume to local folder",
+	Long: `Incrementally backup volume data.
+	
+	It is expected that you use this inside a script, to loop through
+	all possible volume ids that needs to be backup to local folder.
+	
+	The volume id does not need to exist locally or even remotely.
+	This will help to backup future new volumes.
+	
+	Usually backing up is just copying the .dat (and .idx) files.
+	But it's tricky to incremententally copy the differences.
+	
+	The complexity comes when there are multiple addition, deletion and compaction.
+	This tool will handle them correctly and efficiently, avoiding unnecessary data transporation.
+  `,
+}
+
+func runBackup(cmd *Command, args []string) bool {
+	if *s.volumeId == -1 {
+		return false
+	}
+	vid := storage.VolumeId(*s.volumeId)
+
+	// find volume location, replication, ttl info
+	lookup, err := operation.Lookup(*s.master, vid.String())
+	if err != nil {
+		fmt.Printf("Error looking up volume %d: %v\n", vid, err)
+		return true
+	}
+	volumeServer := lookup.Locations[0].Url
+
+	stats, err := operation.GetVolumeSyncStatus(volumeServer, vid.String())
+	if err != nil {
+		fmt.Printf("Error get volume %d status: %v\n", vid, err)
+		return true
+	}
+	ttl, err := storage.ReadTTL(stats.Ttl)
+	if err != nil {
+		fmt.Printf("Error get volume %d ttl %s: %v\n", vid, stats.Ttl, err)
+		return true
+	}
+	replication, err := storage.NewReplicaPlacementFromString(stats.Replication)
+	if err != nil {
+		fmt.Printf("Error get volume %d replication %s : %v\n", vid, stats.Replication, err)
+		return true
+	}
+
+	v, err := storage.NewVolume(*s.dir, *s.collection, vid, storage.NeedleMapInMemory, replication, ttl)
+	if err != nil {
+		fmt.Printf("Error creating or reading from volume %d: %v\n", vid, err)
+		return true
+	}
+
+	if err := v.Synchronize(volumeServer); err != nil {
+		fmt.Printf("Error synchronizing volume %d: %v\n", vid, err)
+		return true
+	}
+
+	return true
+}