Add a diagonal="label" option to scatter_matrix [enhancement whish]
Instead of just histograms or kde, i would like to be able to ask scatter_matrix to put the name of the series in the diagonal panels. 

Below is an example of what i did "manually". just notice how the diagonal panel is used to give the name of the series (in that case i also renamed the series to something descriptive before plotting, but that is the responsibility of the user of course)

![scatter_policies_mumbai](https://cloud.githubusercontent.com/assets/10272606/15898097/ece49042-2d64-11e6-9822-d447963abae4.png)
