Many cleanups to Transaction, see detailed.

Default-ize the sequence rather than use a number, and default to bytes
for input. I doubt anybody ever uses this anyways.

Remove weird convenience code, and remove wallet logic. Checking a TX's
affects on a wallet should be managed by the wallet object.

Remove parsing for the weirder SIGHASH types. People use this library
for creating SIGHASH_ALL transactions, and I don't see the need to
support these other types at the moment since this library's more used
for wallets than for hardcore bitcoin tx analysis/creation. They weren't
tested anyways.

Add note about potentially improving performance by providing
pubkey/address. Deriving from the private key is slower, that
information should probably be cached by the end user.
add back SIGHASH code to transaction
Merge pull request #76 from coinpunk/transactioncleanups

Many cleanups to Transaction, see detailed.