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_id.go

@@ -0,0 +1,18 @@
+package storage
+
+import (
+	"strconv"
+)
+
+type VolumeId uint32
+
+func NewVolumeId(vid string) (VolumeId, error) {
+	volumeId, err := strconv.ParseUint(vid, 10, 64)
+	return VolumeId(volumeId), err
+}
+func (vid *VolumeId) String() string {
+	return strconv.FormatUint(uint64(*vid), 10)
+}
+func (vid *VolumeId) Next() VolumeId {
+	return VolumeId(uint32(*vid) + 1)
+}