ENH: support kind and na_position kwargs in Series.sort_index
closes #13589
closes #13729

I'm waiting on adding the whatsnew entry.  I'm not sure if this is a BUG (the [documentation](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.sort_index.html) says these parameters exist) or an ENH (we're adding support for these parameters).  It depends on how you look at it!
