Add a univariate MA model

[bashtage]

Add a pure moving average model for the mean.

[vchernat]

Why not add a general ARFIMA(p,q) model just as it is done in rugrach in R?

[bashtage]

@vchernat I don't generally see much of a point in ARFIMA in the types of data I am interested in. It is also relatively difficult to write reliable estimators that work well across a wide range of data for the full ARFIMA with ARCH-errors IME.