Add support for listtransactions RPC request.

This change adds support for the listtransactions RPC command.  To
properly reply to this command, additonal information about received
transactions was added, and is now saved in an account's tx.bin file.
Additionally, when sending a transaction, a *tx.SendTx is now saved to
the Tx store, and is included in listtransactions replies under the
"send" category.

WARNING: All account's tx.bin and utxo.bin files should be removed
before running with this change, or else the files may not be read
correctly.  Removing tx.bin is not an issue as it was not being used
before, and was being saved with incorrect data.  Removing utxo.bin is
not an issue as it will just trigger a rescan on next start.  File
format versions are now included in both files, so automatic updates
from previous file formats will be possible with future changes.

Fixes #12.