Work on implementing listTransactions and listAccounts. they mostly work but are not fully documented and listTransactions does not handle all of the possible types of transactions the bitcoin daemon may report. Aside from adding support for the other types of transactions, maybe add an empty UnknownTransaction constructor to WalletTransaction and return that as a last resort instead of throwing an error.

Since there's already a distinct Transaction type in Mining.hs, the new type for listTransactions's output is called WalletTransaction, but this feels a bit inelegant and it may still change.