add bins argument to histogram

add bins to histrogram
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)
-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 parser from ParserTests
 to TestPythonParser and check that they raise ValueErrors when run
 under other engines