Segwit implementation

[oleganza]

Initial work done by @mgpnd in #25, this is a refactoring follow-up.

[oleganza]

From #25 (comment):

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.