fixes # 40 - allows SIGHASH_* in Tx.is_signature_ok

This change maintains backward compatibility with `Tx.is_signature_ok` by providing a default argument. I'm not convinced this is the best approach, but it's probably better than reimplementing `Tx.is_signature_ok` in an external function.

This is a four-line change in the core sources. The rest are either comments or new tests.