Add support for partially signed tx, closes echovl#38

* allow to set additional witnesses for partially signed tx preparation
* change calculateMinFee to consider additional witnesses that will
  increase the tx length. It is still not clear how to precisely
  calculate the bytes for witnesses, in this implementation
  for each additional witnesses there will be an additional 100 bytes,
  (32 public key, 64 signature, 4 index/key in cbor). Those are not
  actually enough so we are considering 1 additional byte
  for each additional witness after the first.

tx_builder.go

@@ -15,6 +15,8 @@ type TxBuilder struct {
 	pkeys    []crypto.PrvKey
 
 	changeReceiver *Address
+
+	additionalWitnesses uint
 }
 
 // NewTxBuilder returns a new instance of TxBuilder.
@@ -48,6 +50,12 @@ func (tb *TxBuilder) SetFee(fee Coin) {
 	tb.tx.Body.Fee = fee
 }
 
+// SetAdditionalWitnesses sets future witnesses for a partially signed transction.
+// This is useful to compute the real length and so fee in advance
+func (tb *TxBuilder) SetAdditionalWitnesses(witnesses uint) {
+	tb.additionalWitnesses = witnesses
+}
+
 // AddAuxiliaryData adds auxiliary data to the transaction.
 func (tb *TxBuilder) AddAuxiliaryData(data *AuxiliaryData) {
 	tb.tx.AuxiliaryData = data
@@ -143,6 +151,14 @@ func (tb *TxBuilder) MinCoinsForTxOut(txOut *TxOutput) Coin {
 func (tb *TxBuilder) calculateMinFee() Coin {
 	txBytes := tb.tx.Bytes()
 	txLength := uint64(len(txBytes))
+	// for each additional witnesses there will be an additional 100 bytes,
+	// (32 public key, 64 signature, 4 index/key in cbor)
+	txLength += uint64(tb.additionalWitnesses * 100)
+	// apparently that is not enough, so just consider 1 additional byte
+	// for each additional witness after the first
+	if tb.additionalWitnesses > 1 {
+		txLength += uint64(tb.additionalWitnesses - 1)
+	}
 	return tb.protocol.MinFeeA*Coin(txLength) + tb.protocol.MinFeeB
 }