[mercadobitcoin] Mercado Bitcoin (almost) full implementation.

Related issue: #784

Full implementation with examples and tests. It is missing only the generic com.xeiam.xchange.service.polling.PollingTradeService#getTradeHistory function, but it is possible to access the information using the raw service com.xeiam.xchange.mercadobitcoin.service.polling.trade.MercadoBitcoinTradeServiceRaw#getMercadoBitcoinUserOrders method.