[Poloniex] expose trade response DTO; exception handling

*  Made `PoloniexTradeResponse` DTO available via `buy()`/`sell()` raw methods (*will break clients expecting a `String`*)
* Catch non-200 HTTP response via `PoloniexException` on the public API methods
* `PoloniexTradeResponse` & `PoloniexException` were not working correctly together to catch HTTP-2xx API errors
  * `PoloniexTradeResponse` is immutable
      * Remove Setters -- use `@JsonCreator` constructor
  * Throw `ExceptionalReturnContentException` to parse JSON error into `PoloniexException`