Add kwargs for kde in scatter_matrix
Minor change to allow customizing the kde through the pandas api.

Have you seen [seaborn](http://stanford.edu/~mwaskom/software/seaborn/tutorial/axis_grids.html#grid-tutorial)? It handles this type of plotting much better than pandas. Specifically, this plot is covered by [`pairgrid` / `pairplot`](http://stanford.edu/~mwaskom/software/seaborn/tutorial/axis_grids.html#plotting-pairwise-relationships-with-pairgrid-and-pairplot)
 @TomAugspurger good point. Yes, I use have used it but I often forget to use it.
 @TomAugspurger status of this?
 @TomAugspurger close?
