improve Transaction._estimateFeePerKb() precision

Use the binary interpretation of kilobyte (1024 bytes) and return the
correct amount of satoshis.

Author: lpinca

lib/transaction/transaction.js

@@ -985,11 +985,11 @@ Transaction.prototype._clearSignatures = function() {
 };
 
 Transaction._estimateFee = function(size, amountAvailable, feePerKb) {
-  var fee = Math.ceil(size / 1000) * (feePerKb || Transaction.FEE_PER_KB);
+  var fee = Math.ceil((size / 1024) * (feePerKb || Transaction.FEE_PER_KB));
   if (amountAvailable > fee) {
     size += Transaction.CHANGE_OUTPUT_MAX_SIZE;
   }
-  return Math.ceil(size / 1000) * (feePerKb || Transaction.FEE_PER_KB);
+  return Math.ceil((size / 1024) * (feePerKb || Transaction.FEE_PER_KB));
 };
 
 Transaction.prototype._estimateSize = function() {