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`.