Support 'interpolation' keyword in pandas.DataFrame.quantile and pandas.Series.quantile

[rhkarls]

interpolation : {‘linear’, ‘lower’, ‘higher’, ‘midpoint’, ‘nearest’}

This would make it consistent with numpy.percentile options (new in 1.9). See http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html

[jreback]

seems straightforward to pass this kw to np.percentile. Want to do a PR?

[jreback]

xref #10207 . Let's do this in a separate PR though.