Code restructuring, assigning proper subsystems to each

- Drivers contain
    * donut/*
    * file/*
    * memory/*

 - Storage format contains
    * donut/*
      - GetObject() --> renamed to GetObjectReader()
      - Deleted stale objectwriter.go, renamed donutwriter.go to object_writer.go

Simplifying, and documenting codebase further

pkg/storage/donut/bucket.go

@@ -0,0 +1,15 @@
+package donut
+
+type donutBucket struct {
+	nodes   []string
+	objects map[string][]byte
+}
+
+// GetNodes - get list of associated nodes for a given bucket
+func (b donutBucket) GetNodes() ([]string, error) {
+	var nodes []string
+	for _, node := range b.nodes {
+		nodes = append(nodes, node)
+	}
+	return nodes, nil
+}