categories grouping needed for tools.plotting.scatter_matrix
I found scatter_matrix super useful to discover correlations between variables. However, it doesn't support distinguishing different categories of points. Check check [this](http://trellischarts.com/images/Scatter-Plot-Matrix-Window.png) out. 

I've modified scatter_matrix a a bit so it supports this functionality. See [this](http://nbviewer.ipython.org/github/tikazyq/stuff/blob/master/scatter_matrix_test.ipynb) or below images to check the effect.

![png-1](https://cloud.githubusercontent.com/assets/3393101/5055673/5f0df37a-6c5f-11e4-9393-2003f6af6ea6.png)

![png](https://cloud.githubusercontent.com/assets/3393101/5055675/6f2ba8ce-6c5f-11e4-8128-4c30c8ab387b.png)
