Add linear map scale to footer section

for #1962 (and others)
* displays user friendly units
* localizes to imperial or metric

I wanted it to be really unobtrusive and blend in with the other things on the bottom of the screen (contributors, api status, etc).

![screenshot 2014-06-20 18 06 19](https://cloud.githubusercontent.com/assets/38784/3346533/8dee4570-f8c7-11e3-8c3f-4e111205b13a.png)

thoughts?