BUG: pivot fixing

BUG: Fix pivoting with ``nan`` in the index (GH3558_)

CLN: removed combine_from_indexers (not necessary) in lib.pyx

BUG: fixed unstack with an index with out any values for an entry
Merge pull request #3627 from jreback/multi_nan

BUG: (GH3588) fix pivoting with nan in the index