Renzo/external pr bar

I added a slim gray bar beneath the legend to help explain that the small dark bars in the aging chart correspond to external pull requests. It looks like [this](https://www.dropbox.com/s/zdyxda7gftxj4xh/Screenshot%202014-07-12%2022.49.34.png).

I also added the text "Updated" to the timestamp in the legend, and updated the README to explain usage of `wall.py`.
Seems a shame to have to go to this length to get one more bar.  Maybe we can build the whole legend with SVG?  Or we could skip the d3 calls, and use literal svg for this bar down below.