[TASK] Add balance_change array to transaction response, computed from metadata

I spoke with Geert today and we are planning on having a function called getBalanceChanges on the Meta class that returns an array with objects of the form you suggested:
    { "account": "rAsdf", "balance_change": AmountObject }
And adding a toJSON method to the Amount class that serializes amounts as
    {
        "currency": "XRP",
        "issuer": "",
        "value": "0.01"
    }
Then to drops to XRP conversion only happens if you JSON stringify. Does that sound good?