Remove the special case for empty dataframes in to_html.

Fixes issue #6062.
BUG/API: allow TimeGrouper with other columns in a groupby (GH3794)
CLN/API: replace groupby.CustomGrouper with Grouper
         rename internally Grouper to BaseGrouper to avoid conflict
         TimeGrouper to now inherit from Grouper
DOC: update groupby docs for using pd.Grouper
CLN: refactor of groupby/resample to handle Grouper
     in a more elegant / cleaner way by keeping internal
     groupby state inside the Grouper rather than passing
     around lots of results

DOC: minor doc edits for groupby.rst / v0.14.0
PEP8: minor pep changes
Merge pull request #6405 from filmor/patch-1

Remove the special case for empty dataframes.
API: A tuple passed to DataFame.sort_index will be interpreted as the levels of
     the index, rather than requiring a list of tuple (GH4370)
ENH: Have pivot and pivot_table take similar arguments
Merge pull request #5505 from jsexauer/fix5505

ENH: Have pivot and pivot_table take similar arguments
DEPR: remove 'columns' kw from DataFrame.sort (prior deprecated circa 2012)
Merge pull request #6516 from jreback/time_grouper

BUG/API: allow TimeGrouper with other columns in a groupby (GH3794)
Merge pull request #6639 from jreback/sort_error

API: A tuple passed to DataFame.sort_index will be interpreted as the levels of the index (GH4370)
DOC: whatsnew edits
Speed up DatetimeConverter by using Matplotlib's epoch2num when possible.
TST: skip sql tests if connection to server fails
Merge pull request #6651 from jorisvandenbossche/sql-skiptests

TST: skip sql tests if connection to server fails
Added vbench performance benchmark for speed up of DatetimeConverter.
SQL: move df.to_sql to generic + update with new functionality
Merge pull request #6598 from jorisvandenbossche/sql-to_sql

SQL: move df.to_sql to generic + update with new functionality
BUG: disallow mixed dtype operations