ENH: Use Welford's method in stats.moments.rolling_var

This PR implements a modified version of Welford's method to compute
the rolling variance. Instead of keeping track of the sum and sum of
the squares of the items in the window, it tracks the mean and the sum
of squared differences from the mean. This turns out to be (much) more
numerically stable.

The formulas to update these two variables when adding or removing an
item from the sequence are well known, see e.g.

http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance

The formulas used when both adding one and removing one item I have
not seen explicitly worked out anywhere, but are not too hard to come
up with if you put pen to (a lot of) paper.
add bins argument to histogram

add bins to histrogram
Merge pull request #6817 from jaimefrio/rolling_var-welford

ENH: Use Welford's method in stats.moments.rolling_var
CLN: failing test on 2.6/windows re GH6866, CustomBusinessMonth
DOC: release/v0.14.0.txt edits
Merge pull request #6850 from zachcp/histograms

DOC: add bins argument to Histogram function
BUG: pass bins arg in hist_frame
Merge pull request #6935 from TomAugspurger/hist-fix

BUG: pass bins arg in hist_frame
API: change AssertionError to TypeError for invalid types passed to concat (GH6583)
REGR: TextFileReader in concat, which was affecting a common user idiom (GH6583)
Merge pull request #6941 from jreback/concat

REGR/API: accept TextFileReader in concat (GH6583)
BUG/ENH: Warn when falling back to python parser

-raise ValueError when engine='c' specified with unsupported options
-raise ValueError when fallback to python causes options to be ignored
-produce ParserWarning on fallback to python when no options ignored
-fix bug in C parser with leading whitespace and \r-delimited files
 (add test)
-translate sep='\s+' to delim_whitespace=True (add test)
-raise ValueError if the user specifies both `sep` and
 `delim_whitespace=True`
-specify engine='python' in tests with sep='\s+' and multiindex column
 input (work around GH 6893)
-add 'engine' option to docstring of read_csv and read_table
-copy tests that previously fell back to python from ParserTests to
 TestPythonParser and check that they raise ValueErrors when run
 under other engines
Merge pull request #6889 from mcwitt/fix-gh6607

BUG/ENH: Add fallback warnings and correctly handle leading whitespace in C parser
BUG: Series/DataFrame.rank() doesn't handle small floats correctly #6868

adding test for ranking with np.inf

Added release note #6886

Fixing float conversions in test_rank()
Update indexing.py

Adding a space
Merge pull request #6946 from ribonoous/patch-1

Update indexing.py
BLD: add excel deps to 2.6 build
BUG: Fix to read decimal seconds from Excel.

Fix to allow decimal seconds to be read from Excel dates and times
into datetime objects. #5945.