code: function signWithLocalPrivateKey(wallet_id, account, is_change, address_index, text_to_sign, handleSig){
  	var path = "m/44'/0'/" + account + "'/"+is_change+"/"+address_index;
  	var privateKey = xPrivKey.derive(path).privateKey;
  	var privKeyBuf = privateKey.bn.toBuffer({size:32}); // https://github.com/bitpay/bitcore-lib/issues/47
  	handleSig(ecdsaSig.sign(text_to_sign, privKeyBuf));
  }
docs: sign with local privateKey
  @param {string} wallet_id - wallet id
  @param {number} account - account index
  @param {number} is_change - is_change is 0 not is_change is 1
  @param {number} address_index - address index
  @param {string} text_to_sign - text
  @param {function} handleSig - callback