Do not duplicate sending final rescan tx.
Send nil when rescan is finished.

I previously fixed the duplicate send (before seeing GH issue #54),
but forgot that btcwallet expects a nil reply when rescan has
finished.  This adds the final reply back, but replies with nil.

Fixes #54.
Search each txout for payment to a wallet address.

This switches a break to a continue if a txout does not include a
pay-to-pubkey-hash script type.  btcwallet only supports
pay-to-pubkey-hash at the moment, and this fixes an issue where a tx
may have an different type of output, as well as pay-to-pubkey-hash,
which may be ignored by the wallet notification code.

Found by dhill.
Update README.md now that Go 1.2 has been released.
Make authentication check time-constant.
Correctly set vout for getrawtransaction.

The vout field (as part of the getrawtransaction JSON reply) should be
set to the input's previous outpoint's index, not the current input
index.

Found by flam and reported on IRC. Thanks!