Fix DataFrame.to_latex() with MultiIndex columns

Currently, the positioning of \midrule is determined by the number of index
levels, not columns levels:

	>>> print pd.DataFrame({('x', 'y'): ['a']}).to_latex()
	\begin{tabular}{ll}
	\toprule
	{} &  x \\
	\midrule
	{} &  y \\
	0 &  a \\
	\bottomrule
	\end{tabular}

The fix is simple: use the number of column levels instead of the number
of index levels.
BUG: rolling/expanding_* treamtment of center
ENH: plot functions accept multiple axes and layout kw
ENH: Add duplicated/drop_duplicates to Index
API: consistency in .loc indexing when no values are found in a list-like indexer (GH7999)
BUG: Area plot legend has incorrect color
BUG: related to GH5080, get_indexer choking on boolean type promotion (GH8024)
Merge pull request #8026 from jreback/series_map

BUG: related to GH5080, get_indexer choking on boolean type promotion (GH8024)
Merge pull request #8003 from jreback/indexing

API: consistency in .loc indexing when no values are found in a list-like indexer GH7999)
CLN: add html_encoding path to setup.py data
BUG: fix html reading for bytes types in py3
TST: add assertion that we will actually read a file
DOC: release notes [ci skip]
Merge pull request #8030 from cpcloud/read-html-py3-encoding-7927

BUG: fix py3 read_html bytes input
DOC: suppress warning + fix reshape example
Merge pull request #8034 from jorisvandenbossche/doc-fixes4

DOC: suppress warning + fix reshape example
Merge pull request #8027 from sinhrks/area_legend

BUG: Area plot legend has incorrect color
Merge pull request #7979 from sinhrks/dup_idx

ENH: Add duplicated/drop_duplicates to Index
Fixing 'chunksize' parameter name typo io.rst
Merge pull request #8035 from nesanders/patch-1

DOC: Fixing 'chunksize' parameter name typo io.rst