BUG: GH2763 fixed downcasting of groupby results on SeriesGroupBy
      e.g. was returning float64 on int64 input when possible to preserve
      in first/last
DOC: more cookbook examples
Merge pull request #3153 from jreback/cook6

DOC: more cookbook examples
Merge pull request #3152 from jreback/GH2763

BUG: GH2763 fixed downcasting of groupby results on SeriesGroupBy
SCR: format tweaks to test_perf
BLD: speed up test_perf.sh
ENH: Pass **kwargs through to matplotlib .scatter()
Merge pull request #3159 from danbirken/kwargs_to_scatter_plot

ENH: Pass **kwargs through to matplotlib .scatter()
DOC: Add more detailed description of commit prefixes
Merge pull request #3160 from danbirken/commit_prefix_explain

DOC: Add more detailed description of commit prefixes