* 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
In order to add convenience methods to a transaction, such as pushing new
inputs and outputs, we need to first have the notion of an initialized
transaction, which is actually not blank. An initialized transaction just has
default values for everything, such as no inputs and no outputs, and default
version and nlocktime.
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);