Add Box-Cox transformation parameter to ARIMA models and forecasts

[signaflo]

The ARIMA models should be able to automatically apply a supplied one parameter Box-Cox transformation when fitting the model.

When the model is forecast, the Box-Cox transformation should be reversed in order to get forecasts on the scale of the original data.

For basic use cases and explanation, see here: https://www.otexts.org/fpp/2/4