Missing properties on Order, Transaction

[rgamage]

It seems like IOrder and ITransaction are missing several properties that are returned by the Oanda API, but are not available in this library because the models are missing those properties. A few of these would be very useful.

Instrument, Units, and Price would all be welcome additions to the IOrder and ITransaction models.

[rgamage]

I see now that these 'missing' properties are, in fact, there on other implementations of the IOrder and ITransaction interfaces, like StopOrder, StopLossOrder, etc. so this issue could be closed.