BUG: Fix to_datetime to properly deal with tz offsets #3944

Currently for certain formats of datetime strings, the tz offset will
just be ignored.
DOC: Clarify that DataFrame string formatters must return strings

I am not 100% sure that string formatters must really return *unicode* strings
(non-unicode strings seem to work, too), but I am leaving that because it's
what is already documentated.

Fixes #6052.
Merge pull request #6084 from jacobschaer/master

Added 'Experimental' to Docstrings For GBQ
DOC: supress warnings with okwarning in whatsnew
BUG: array_equivalent was not passing its own doctest. The examples showed lists being passed to array_equivalent. array_equivalent expects ndarrays only.
Merge pull request #6088 from unutbu/fix-equivalent

BUG: array_equivalent was not passing its own doctest.
Merge pull request #6086 from jorisvandenbossche/doc-warnings

DOC: suppress warnings with okwarning in whatsnew
DOC/BLD: ipython_directive cleanups
BUG: ensure plt is imported before it's used in hist_frame
Merge pull request #6094 from dsm054/reorder-hist_frame-import

BUG: ensure plt is imported before it's used in hist_frame
DOC: remove warnings in panel.apply docs (GH6087)
TST: disable get_components_yahoo tests as unreliable
DOC: beef up tutorial section. GH5837
Merge pull request #6095 from y-p/PR_tuts

DOC: beef up tutorial section. GH5837
TST: don't compare inferred index freq on some tests as it may not be preserved
     (which is correct), (GH6093)
Merge pull request #6089 from jreback/apply_warn

DOC: remove warnings in panel.apply docs (GH6087)
Merge pull request #6097 from jreback/hdf_fix

TST: don't compare inferred index freq on some tests as it may not be preserved (which is correct)
BUG: not converting scalars properly to M8/m8 on assignment (GH6079)
CLN: removed need for SNDArray ; simplied SeriesGroupber / SeriesBinGrouper
TST: skip read_dta2 on non-little endian