message: add a way to create a fake signature
diff: @@ -6,7 +6,7 @@ use raw_cbor::{self, de::RawCbor, se::{Serializer}};
  use config::{Config};
  use redeem;
  -use hdwallet::{Signature, XPub, XPrv};
  +use hdwallet::{Signature, XPub, XPrv, XPUB_SIZE, SIGNATURE_SIZE};
  use address::{ExtendedAddr, SpendingData};
  use coin::{Coin};
  @@ -78,6 +78,11 @@ impl fmt::Display for TxInWitness {
  }
  }
  impl TxInWitness {
  + /// this is used to create a fake signature useful for fee evaluation
  + pub fn fake() -> Self {
  + let fakesig = Signature::from_bytes([0u8;SIGNATURE_SIZE]);
  + TxInWitness::PkWitness(XPub::from_bytes([0u8;XPUB_SIZE]), fakesig)
  + }
  /// create a TxInWitness from a given private key `XPrv` for the given transaction id `TxId`.
  pub fn new(cfg: &Config, key: &XPrv, txid: &TxId) -> Self {
  