Okcoin China/International

Implementation for okcoin.cn and okcoin.com. Few remarks:

- trade API has only been tested for okcoin.cn
- switch between okcoin.cn and okcoin.com (intl) with:
```
spec.setExchangeSpecificParametersItem("Use_Intl", true);
```
- account balance behaves just like bitvc: available = your main wallet + frozen - borrowed