Refactored axes.violinplot into cbook.violin_stats (arranges violin plot data for drawing), axes.violin (draws pre-arranged violin plot data), and axes.violinplot (uses cbook.violin_stats to draw violin plots via axes.violin)
Updated whats_new.rst.
Merge remote-tracking branch 'mpl/master' into vp-squashe

Conflicts:
	doc/users/whats_new.rst
	lib/matplotlib/tests/test_axes.py
	lib/matplotlib/tests/test_mlab.py