Add interest rate to instrument

[Martin Seeler]

An Instrument provided by Oanda has also informations about their interest rates, which can be useful for carry trades. The field is requested in the current request, but ignored by out case class.

The response from Oanda looks like this:

{
  "instrument": "ZAR_JPY",
  "displayName": "ZAR/JPY",
  "pip": "0.01",
  "precision": "0.001",
  "maxTradeUnits": 10000000,
  "maxTrailingStop": 10000,
  "minTrailingStop": 5,
  "marginRate": 0.02,
  "halted": true,
  "interestRate": {
    "ZAR": { // base currency
      "bid": 0.06,
      "ask": 0.0718
    },
    "JPY": { // quote currency
      "bid": -0.0015,
      "ask": 0.0025
    }
  }
}

ZAR in this case is the base currency, JPY the quote currency. I'm still not sure how the representation should look like.