* Refactor transaction into a different subfolder
* Added a lot of tests for sighash and transaction serialization (from
reddit's and Ryan X. Charles' `fullnode`)
* Drop "only" from sighash tests and consolidate logs
For Transaction, Block and Blockheader. This is a convenience so if you happen
to have the buffer for one of these, you can make a new one like this:
Transaction(txbuf);
Rather than having to do this:
Transaction().fromBuffer(txbuf);