add HDKey.sign/HDKey.verify methods

lib/hdkey.js

@@ -152,6 +152,14 @@ HDKey.prototype.deriveChild = function (index) {
   return hd
 }
 
+HDKey.prototype.sign = function (hash) {
+  return secp256k1.sign(hash, this.privateKey).signature
+}
+
+HDKey.prototype.verify = function (hash, signature) {
+  return secp256k1.verify(hash, signature, this.publicKey)
+}
+
 HDKey.prototype.toJSON = function () {
   return {
     xpriv: this.privateExtendedKey,