The signature targets should contain tx's parents

[heeeeeng]

The tx's parents are not contained by signature targets.
A malicious node is able to change a tx's parents and broadcast the tampered tx to the network.
This will lead to a double spending problem, because a natural node can't figgure out which tx is the correct one as these two txs have the same signature.

To solve this, the SignatureTargets() function of each tx type should be changed:

func (t *Sequencer) SignatureTargets() []byte {
	var buf bytes.Buffer

	panicIfError(binary.Write(&buf, binary.BigEndian, t.AccountNonce))
	panicIfError(binary.Write(&buf, binary.BigEndian, t.Issuer.Bytes))
	panicIfError(binary.Write(&buf, binary.BigEndian, t.Height))

	return buf.Bytes()
}

Tx parents information should be added.

Another necessary changing is the order of tx creation. In current version, the tx is been signed first and then been assigned which txs are it's parents. The problem is that this will make it impossible for SignatureTarget() function to get parents information as parents are known only after signing. The solution changing the order: assign parrents first and then sign the tx with parents information.