allow scripthash & multisig to be signed more than once
don't append serialized script if it's already there
sometimes scriptSig.chunks has length 0
fix bug where you had to sign with 3rd key first
bring p2sh multisig in line with BIP 0011

1) remove placeholders when a p2sh multisig transaction has been fully signed.
2) prepend the script with OP_0
https://github.com/bitcoin/bips/blob/master/bip-0011.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki