Implement segwit transaction serialization

[mgpnd]

Usage example for spending from 2-of-2 multisig P2SH-P2WSH address.

tx = BTC::SegwitTransaction.new
tx.outputs = outputs
tx.inputs = inputs
tx.inputs.each do |txin|
  txin.signature_script = input_redeem_script.data
  txin.witness =  ["".b, signature1, signature2, input_witness_script.data]
end

[oleganza]

Awesome, thank you.

A few comments:

1. You can use BTC.hash256() instead of BTC.sha256(BTC.sha256()).
2. New signature_hash function uses dup on the transaction. But current version of dup does not duplicate witness field. This is not a bug for sighash, but should be addressed as soon as a subclass is introduced. (That is, lets not forget the new field txin.witness.)
3. I'd recommend not creating a subclass at all. I'd rather extend BTC::Transaction with segwit serialization and sighash options, in the same way BTC::TransactionInput simply adds witness.

I would be glad to help with refactoring this. Maybe lets merge this on a segwit branch in this repo first and then refactor there?

[oleganza]

Merged to a segwit branch to do some design changes before moving to master.