[HitBTC] OrderBook is missing support of timestamp

[dp770]

Documentation: https://api.hitbtc.com/#order-book
Stream URL: https://api.hitbtc.com/api/2/public/orderbook/ETHBTC
Example message:
{ "ask": [ { "size": "0.088", "price": "0.046002" }, { "size": "0.200", "price": "0.046800" } ], "bid": [ { "size": "0.005", "price": "0.046001" }, { "size": "0.200", "price": "0.046000" } ], "timestamp": "2018-11-19T05:00:28.193Z" }