EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.
Merge pull request #5231 from unutbu/nan-sort

EHN/FIX: Add na_last parameter to DataFrame.sort. Fixes GH3917
COMPAT: change Series.sort to use na_position rather than deprecated na_last
BUG: shift now converts named axis to numer. fixes #6724
TST: test named shift with named axis
Merge pull request #6725 from dalejung/shift_named_axis

Shift named axis