Implement BTCChina getMarketDepths API method.

[getMarketDepth2](http://btcchina.org/api-trade-documentation-en#getmarketdepth2) - Get the complete market depth. Returns all open bid and ask orders.
> They're not useless, as even the single word @params allow IDEs to show JavaDoc information during code assisting

What are you talking about?

> The empty @return param is useless, but having it there encourages developers to fill it in.

Apparently it's not effective.

Doesn't Eclipse show a warning at each empty `@param`?