Add ability to create scriptSigs with empty signatures

For incomplete scriptSig OP_0 is used as a signature placeholder. 

Pay-to-address: `OP_0 [pubkey]`
Pay-to-pubkey: `OP_0`
P2SH with 2-of-3 CHECKMULTISIG: `OP_0 OP_0 OP_0 [redeem script]`

The plan is to fill unsigned tx inputs with such scripts, so that transaction signers may update these scripts with proper signatures.