Kde kwargs

Adds ability to set the bandwidth used for the kde and the indices at which it is evaluated. The first is passed to the bw_method kwarg for scipy.stats.gaussian_kde (for scipy >= 0.11.0); the second is passed to gkde.evaluate. For the case when scipy < 0.11.0 but bw_method is specified, bw_method is ignored (not passed to scipy.stats.gaussian_kde), but a warning is issued to alert the user. 

https://github.com/pydata/pandas/issues/4298