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/storage/volume_version.go

@@ -0,0 +1,9 @@
+package storage
+
+type Version uint8
+
+const (
+	Version1       = Version(1)
+	Version2       = Version(2)
+	CurrentVersion = Version2
+)