Add a distance legend

Fixes #31.

![image](https://cloud.githubusercontent.com/assets/924374/4069147/74db730a-2e41-11e4-8364-ed814c480328.png)

Live demo here:
http://demo.kotachrome.com:3001/

One problem I know about is that using the browser zoom will make the distance legend inaccurate.  It also may be inaccurate on high-DPI displays such as Retina Macs.  I'm opening this for feedback before I get too carried away fixing that.