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.
There is no warning for this `@param` line. Only warnings for empty `@return` and `@link` after `@see`.
But the empty `@param` is useless, it is duplication with the method parameter list, I think.