add fee estimation method to txp

lib/model/txproposal.js

@@ -169,6 +169,25 @@ TxProposal.prototype.getRawTx = function() {
   return t.uncheckedSerialize();
 };
 
+TxProposal.prototype.estimateFee = function(walletN) {
+  // Note: found empirically based on all multisig P2SH inputs and within m & n allowed limits.
+  var safetyMargin = 0.05;
+
+  var walletM = this.requiredSignatures;
+
+  var overhead = 4 + 4 + 9 + 9;
+  var inputSize = walletM * 72 + walletN * 36 + 44;
+  var outputSize = 34;
+  var nbInputs = this.inputs.length;
+  var nbOutputs = (_.isArray(this.outputs) ? this.outputs.length : 1) + 1;
+
+  var size = overhead + inputSize * nbInputs + outputSize * nbOutputs;
+
+  var fee = this.feePerKb * (size * (1 + safetyMargin)) / 1000;
+
+  // Round up to nearest bit
+  this.fee = parseInt((Math.ceil(fee / 100) * 100).toFixed(0));
+};
 
 /**
  * getTotalAmount