ENH: DataFrame.corr(method='spearman') is cythonized.
ENH: DataFrame.corr(method=spearman) related minor improvements.
CLN: cleaned up _try_cast in core/groupby.py to eliminate cruft
DOC: cookbook updates - added idiom section
Merge pull request #3934 from jreback/GH3920

CLN: cleaned up _try_cast in core/groupby.py to eliminate cruft (GH3920)
Merge branch 'correlations' of https://github.com/jniznan/pandas into jniznan-correlations
PERF: stats_corr a smaller vbsuite
DOC: update release notes