hitbtc: v2 adaptations

[mkutny]

Apparently, hitbtc v2 was partially implemented.

So now:

• fetchOpenOrders() adapted for new endpoint
• I removed child parseOrders() as it throwed an exception on empty array of orders. Base class method handles it just well
• order structure have been changed so I adapted parseOrder() as well
• limits and precisions was not compatible with the doc (market had lot and step properties instead but I kept them for backwards compatibility)
• trading fees added to market

[kroitor]

Awesome! Thx so much!