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)
- [ ] Use statsmodels KDE instead of scipy for more flexibility (ref #22)
- [ ] 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