add Transaction#lockUntil stub

lib/transaction/transaction.js

@@ -279,6 +279,16 @@ Transaction.prototype.fromObject = function(transaction) {
   this.version = transaction.version;
 };
 
+/**
+ * sets nLockTime so that transaction is not valid until 
+ * the desired date or block height
+ * @param {Date | Number} time  
+ */
+Transaction.prototype.lockUntil = function(time) {
+  $.checkArgument(time);
+  this.nLockTime = DEFAULT_NLOCKTIME;
+};
+
 Transaction.prototype.toJSON = function toJSON() {
   return JSON.stringify(this.toObject());
 };