Find better solution than sprintf in lib.Node diff function

[kpelzel]

#140 replaces lib.urlpush with sprintf to avoid a beginning slash. A beginning slash will cause issues with the SME because it filters out any STATE_CHANGE events that contain extensions with a leading slash. This solution is not ideal, but was passed in due to a time crunch.

related to PR: #115

These problems occur in lib.Node.Diff()

	// handle extensions
	for _, u := range eright {
		nodeExt, ok := m.exts[u]
		if !ok {
			r = append(r, lib.URLPush(prefix, u))
			r = append(r, fmt.Sprintf("%s%s", prefix, u))
			continue
		}
		d, _ := lib.MessageDiff(n.exts[u], nodeExt, lib.URLPush(prefix, u))
		d, _ := lib.MessageDiff(n.exts[u], nodeExt, fmt.Sprintf("%s%s", prefix, u))
		r = append(r, d...)
		for i := range eleft {
			if eleft[i] == u {
@@ -399,9 +407,11 @@ func (n *Node) Diff(node lib.Node, prefix string) (r []string, e error) {
		}
	}
	for _, u := range eleft {
		r = append(r, lib.URLPush(prefix, u))
		r = append(r, fmt.Sprintf("%s%s", prefix, u))