INT: add TimeDeltaBlock support in internals

ENH: GH3371 support timedelta fillna
BUG/ENH: add bfill/ffill support for datetime64[ns]
API: provide automatic dtype conversions on _reduce operations
API: exclude non-numerics if mixed types in _reduce operations

BUG: timedelta fixes

CLN: small cleaning in nanops.py

BUG: allow _reduce to call .apply for certain operations when the who block fails
     via a reduce exception
Merge pull request #4684 from jreback/timedelta_fill

ENH: GH3371 support timedelta fillna
DOC: updated release.rst to conform headings
BLD: make test_perf.sh work on OSX too
TST: (GH4550) where with a dup index in Series
BUG: (GH4548) inplace updating of a duplicate Series with a boolean aligning incorrectly
Merge pull request #4779 from jreback/dups_ops

TST/BUG: duplicate indexing ops with a Series using where and inplace add buggy (GH4550/GH4548)