add tooltip to static sigma graph in intro to insure it gets seen

This should definitely be classified as a small thing, but since the focus of widgets is interactivity, I would very much like to make sure readers see that the static sigma graph is static only because we intentionally made it static.  I changed the `img` to use the tooltip functionality from cosmo.css, so that on hover, the tooltip appears with the message that had been below the `img` in `p` in the original.  I only made the change in the markdown and did not rebuild for this pull to isolate the change.

![image](https://cloud.githubusercontent.com/assets/837910/5456287/12d02066-8508-11e4-8838-8dcd522c8b91.png)