Added bw_method to sm.graphics.violinplot + default colors?

Hello there,
I have a homebrewed version of this (also based off of the same code!) but it allows the user to specify the bandwidth method. So in this PR, I added the ability to change the bandwidth method. It's currently with the regular args because I didn't think this is a plot argument, but rather an estimation argument. Suggestions?

Here are some examples: http://nbviewer.ipython.org/gist/olgabot/7915540

Besides the bandwidth method, can the default facecolor also be changed? The yellow is rather ugly, and I'd much prefer something from colorbrewer: http://bl.ocks.org/mbostock/5577023

Thanks,
Olga

PS Initially I was going to add this to `pandas` (https://github.com/pydata/pandas/issues/5676) but since `statsmodels` already has most of the code, this seemed like a more natural choice.