ENH: including offset/freq in Timestamp repr (#4553)
Merge pull request #6575 from rosnfeld/issue_4553

ENH: including offset/freq in Timestamp repr (#4553)
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.