implemented ListReceivedByAccount

refs #51 
Explicitly setting the cap equal to the length is unnecessary.  In fact, I would avoid make() completely, and just use a composite literal:

```
ret := []btcjson.ListReceivedByAccountResult{
    {
        Account:       "",
        Amount:        amount.ToUnit(btcutil.AmountBTC),
        Confirmations: uint64(confirmations),
    },
}