Bitfinex: active positions query

Added Raw method for querying currently active margin positions. Including derived field for order type.

`BitfinexActivePositionsResponse [id=199537, symbol=btcusd, status=ACTIVE, base=476.755, amount=-0.01, timestamp=1.40950848E9, swap=0.0, pnl=-0.0292244, orderType=ASK]`