API: make Series.sort/order arguments in the same order
     drop unused Series.sort order argument
     added Series.sort na_position argument to conform with Series.order
     Series.order default kind is not 'quicksort' to conform with Series.sort
     (and others), (GH6847)
Merge pull request #6848 from jreback/sort

API: make Series.sort/order arguments in the same order