ggplot fixes and made geom_forecast() more general

stat_forecast() should be rarely used as per Hadley's recommendation.
geom_forecast() will now generate forecasts unless specified otherwise.

Fixed geom_forecast(fc) not working with univariate time series. You now
must specify a series when using geom_forecast(fc) with multivariate
series. (Best solution I could find so far).

Removed test functions.