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/sequence/sequence.go

@@ -0,0 +1,7 @@
+package sequence
+
+type Sequencer interface {
+	NextFileId(count uint64) (uint64, uint64)
+	SetMax(uint64)
+	Peek() uint64
+}