Poloniex Full Implementation

Full implementation of the Poloniex Exchange. Also added trade type to Cryptsy's public trades.

Poloniex notes:

Error handling is bad. They return an entirely different JSON structure when there is an error, and I'm not sure how to deal with this.

Poloniex requires a currency pair parameter to cancel an order. Since cancelOrder in PollingTradeService only accepts String parameter, I just used that parameter in conjunction with getOpenOrders to pull the currency pair. This isn't ideal, but Poloniex says that they are working on a solution and we'll only need to send the order id.