Bithumb: Add zero filter for orderbook amount

[thrasher-]

Based off https://api.bithumb.com/public/orderbook/btc, exchange things:

      {
        "price": "148360000",
        "quantity": "0.0002"
      },
      {
        "price": "148371000",
        "quantity": "0.0000"
      },
      {
        "price": "148375000",
        "quantity": "0.0842"
      },

Bithumb will need a filter just like what we do with Bitstamp