Improve kde computation

This addresses several issues with the distribution plots

- The statsmodels density estimate fails on data that isn't double-typed
- The density estimate can become slightly negative (probably due to the FFT-based computation statsmodels uses)

These problems are detailed in #78  and are fixed here. This fix also changes the default `distplot` bin computation slightly by taking the ceiling of the bin number estimate.

This also adds scipy-based kde computation back, paving the way for making statsmodels an optional dependency.

@sjobeek do you want to give this a spin?