fixes #39 - corrects signature_hash for SIGHASH_SINGLE on input index > 0; adds ability to verify legacy SINGLE_HASH transactions; adds TxIn to tx/__init__.py for consistency with Tx and TxOut
added test case for sighash_single
get unit tests to pass for python 3
fixes #39 less intrusively (doesn't require signed long longs in bitcoin_streamer, but "fakes" the "null" value of -1L by using its unsigned equivalent instead)
Merge pull request #40 from mbogosian/master

fixes #39 - corrects signature_hash for SIGHASH_SINGLE on input index > 0; ...