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

The output of parseBalanceChanges gets inserted into the response message which is JSON. The amount object doesn't serialize well to JSON, it dumps a bunch of internal underscore member data. That could be fixed, but then we are back to the same problem as in my last comment.

Is the suggestion to have the JSON response message contain a mixed-type array like this:

    [
        {
            "account": "xxxxx",
            "value": "100"
        }, 
        {
            "account": "xxxxx",
            "currency": "USD",
            "issuer": "yyyyy",
            "value": "10"
        }
    ]

It looks confusing to me. If I'm a newbie ripple-lib open source developer, I would think "is that a bug? what happened to the currency for that first one?"

It would also make parsing more complicated, which is not very friendly for end users. I think the idea is that the balance change information is meant as a convenience for clients of the API and not for internal use by ripple-lib, so it is not necessary to conform to internal data structures of ripple-lib. If ripple-lib ever needs to use balance changes internally it wouldn't be hard to refactor to return Amounts and then do conversion to JSON separately, but that would be premature without a use case.