Remove the special case for empty dataframes in to_html.

Fixes issue #6062.
FIX str.match uses na flag
ENH: Allow longtable in to_latex. Closes #6616.

DOC: Make note about booktabs package.

DOC: Add longtable to release notes.
BUG: Bug in fillna with limit and value specified
Merge pull request #6628 from jreback/fillna_limit

BUG: Bug in fillna with limit and value specified
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
BUG: Error in to_stata when DataFrame contains non-string column names

to_stata does not work correctly when used with non-string names.  Since
Stata requires string names, the proposed fix attempts to rename columns using
the string representation of the column name used.  The main method that
reformats column names was refactored to handle this case.

Patch includes additional fixes for detecting invalid names.

Patch includes some minor documentation fixes.
Merge pull request #6622 from bashtage/stata-timestamps

BUG: Error in to_stata when DataFrame contains non-string column names
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
Added test and basic unicode support

Added network connectivity flag to test for unicode

Fixed Documentation
Merge pull request #6596 from jacobschaer/GBQ_Unicode_Support

Gbq unicode support
Merge pull request #6611 from hayd/str_match_nan

FIX str.match uses na flag
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)