allow configuration of fee per kb

lib/transaction/transaction.js

@@ -634,6 +634,21 @@ Transaction.prototype.fee = function(amount) {
   return this;
 };
 
+/**
+ * Manually set the fee per KB for this transaction. Beware that this resets all the signatures
+ * for inputs (in further versions, SIGHASH_SINGLE or SIGHASH_NONE signatures will not
+ * be reset).
+ *
+ * @param {number} amount satoshis per KB to be sent
+ * @return {Transaction} this, for chaining
+ */
+Transaction.prototype.feePerKb = function(amount) {
+  $.checkArgument(_.isNumber(amount), 'amount must be a number');
+  this._feePerKb = amount;
+  this._updateChangeOutput();
+  return this;
+};
+
 /* Output management */
 
 /**
@@ -831,7 +846,7 @@ Transaction.prototype.getFee = function() {
 Transaction.prototype._estimateFee = function() {
   var estimatedSize = this._estimateSize();
   var available = this._getUnspentValue();
-  return Transaction._estimateFee(estimatedSize, available);
+  return Transaction._estimateFee(estimatedSize, available, this._feePerKb);
 };
 
 Transaction.prototype._getUnspentValue = function() {
@@ -844,12 +859,12 @@ Transaction.prototype._clearSignatures = function() {
   });
 };
 
-Transaction._estimateFee = function(size, amountAvailable) {
+Transaction._estimateFee = function(size, amountAvailable, feePerKb) {
-  var fee = Math.ceil(size / 1000) * Transaction.FEE_PER_KB;
+  var fee = Math.ceil(size / 1000) * (feePerKb || Transaction.FEE_PER_KB);
   if (amountAvailable > fee) {
     size += Transaction.CHANGE_OUTPUT_MAX_SIZE;
   }
-  return Math.ceil(size / 1000) * Transaction.FEE_PER_KB;
+  return Math.ceil(size / 1000) * (feePerKb || Transaction.FEE_PER_KB);
 };
 
 Transaction.prototype._estimateSize = function() {