Merge pull request #40 from mbogosian/master

fixes #39 - corrects signature_hash for SIGHASH_SINGLE on input index > 0; ...
Add Matthew Bogosian to credits.
Merge pull request #42 from mbogosian/master

fixes # 40 - allows SIGHASH_* in Tx.is_signature_ok
Improve automated validation of transactions that use hash types
besides the typical SIGHASH_ALL.