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. Besides that, it'd be nice to leave them, even the `@return` one, to keep consistent with the rest of XChange. The empty `@return` param is useless, but having it there encourages developers to fill it in. If Java8 fails it's build, you can run the mvn commands with the `doclint-java8-disable` profile.