[Poloniex] access to PoloniexTradeResponse DTO's via raw methods
[Poloniex] use @JsonCreator v. getter/setter to throw http-2xx API errors
[Poloniex] throw PoloniexException on http-5xx API errors
[Poloniex] throw PoloniexException as ExchangeException
Merge pull request #733 from nimrood/develop

[Poloniex] expose trade response DTO; exception handling