PERF: use statsmodels kde rather than scipy kde in plotting

[jreback]

pandas already uses statsmodels for various stat functions, this seems natural (and can always fall back to the currernt method of statsmodels is not available)

https://groups.google.com/forum/?fromgroups#!topic/pydata/sQVPKD1RTrs

[jseabold]

I suppose there's no problem if statsmodels is a soft-dependency. We had problems with cyclical dependencies before.

[jreback]

yep..that's how its treated now (its done at run-time) rather than at the file level import
(in this particular case gaussian_kde from scipy is done the same way)

I believe @wesm wanted to use more statsmodels in any event

[jreback]

closed by #4316 (mostly)