add master key reference to hd keys.

lib/bcoin/hd.js

@@ -151,6 +151,8 @@ function HDPriv(options) {
     data = options;
   }
 
+  this.master = options.master || this;
+
   data = this._normalize(data, network.prefixes.xprivkey);
 
   this.data = data;
@@ -323,6 +325,7 @@ HDPriv.prototype.derive = function(index, hard) {
   var privateKey = leftPart.add(new bn(this.privateKey)).mod(ec.curve.n).toArray();
 
   return new HDPriv({
+    master: this.master,
     // version: this.version,
     depth: this.depth + 1,
     parentFingerPrint: this.fingerPrint,