Fixes to geom_forecast()

* Fixed geom_forecast(forecast) when forecast does not have confidence intervals #337
* Improved geom_forecast() handling autoplot.mts() changes (allowing facetting) for using geom_forecast(forecast, series="series1").
* Made fortify.forecast() better handle as.data.frame.forecast() and sanitisation of PI argument.
* Removed unnecessary ggplot grouping when plot.conf=FALSE (as only one line is drawn)