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

Hopefully this fix for issue #39 helps redeem me from the utter embarrassment that is issue #38. :o(

I wrote a [test script](https://gist.github.com/mbogosian/fe35ef43e2cb4e8c2258) to compare the fixed output of `pycoin` with the equivalent output from `bitcoinj`. The python half is adapted as a (now working) unit test in e8b026f.

On a related note, your `BRAIN DAMAGE: this probably doesn't work right` comment was wrong...that part of the code was fine. It just required a number encoding tweak.... :o)