ENH: Add mode method to Series and DataFrame

Abstract building up the tables with counts so value_count and mode can
share. Aww! DOC: Add documentation about mode()
CLN: Mode result needs to be sorted, otherwise hit weirdness with hashing
Merge pull request #5380 from jtratner/add-mode-to-series-and-frame

ENH: Add mode method to Series and DataFrame
Added failing test case for resampling TimeSeries by period with a local timezone.

Related to #5340.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>

Added Test Case for #3609.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>

Fixes Grouping by Period with Timezones

The timestamp generated to partition the data frame doesn't include timezone information, so it was creating the wrong groups.  It also had the frequency ('D') hard coded.

Fixes #5340 and #3609.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
ENH: Always do true division on Python 2.X

Note: this version actually forces `a / b` to always do truediv as well.
It's less complicated to maintain if we do it that way, but it's also
possible to have this `a / b` use div and `a.div(b)` use truediv, just
might be strange. This is definitely different from numpy behavior.

We also may need to add the division future import to more places.
DOC: Add notes about truedivision
Merge pull request #5439 from jtratner/make-div-do-truediv

ENH: Always do true division on Python 2.X
Merge pull request #5432 from kevinastone/master

BUG: fix Resampling a Series with a timezone using kind='period' (GH5430)
Merge pull request #5423 from jmcnamara/enh_excel_multi_index

ENH: Better handling of MultiIndex with Excel
DOC: fix build error in to_latex and release notes
PERF: Fixed decoding perf issue on pyt3 (GH5441)
Merge pull request #5447 from jorisvandenbossche/docs-to_latex-error

DOC: fix build error in to_latex and release notes
Merge pull request #5448 from jreback/hdf_decode

PERF: Fixed decoding perf issue on py3 (GH5441)
DOC: remove docstring of flags attribute during doc building process (#5331)
Merge pull request #5450 from jorisvandenbossche/docs-flags-remove-docstring

DOC: remove docstring of flags attribute during doc building process (#5331)
DOC: small error in cookbook (doc build warning)
Merge pull request #5454 from jorisvandenbossche/patch-1

DOC: small error in cookbook (doc build warning)
BUG: Excel writer doesn't handle "cols" option correctly

Added fix to ensure that Series associated with column
names are handled correctly even if names are duplicated
or swapped. Issue #5427