WIP: Omnibus KDE changes

This PR is for a number of planned changes relevant to KDE

- [x] Rename `sns.violin` to `sns.violinplot` for consistency (closes #34)
- [x] Use statsmodels KDE instead of scipy for more flexibility (ref #12)
- [x] Allow for `sns.kdeplot` to plot bivariate KDE when input data is `n x 2` (closes #40)


Possibly in this or in separate PR

- ~~Addition of higher-level `sns.jointplot` to draw joint distribution with KDE or hexbin and marginal plots~~