Add the metric system

It looks like a few countries use some kind of ki-lo-me-ter system to measure things. I'm not sure why [Queen Elizebeth's 1593 decree of eight furlongs to a mile](https://en.wikipedia.org/wiki/Mile#Statute_mile) isn't universally used, but never-the-less this branch:

- Adds a settings toggle to use metric units
- Updates the distance and speed in the UI to use km and km/h. 
- Uses geocoding to determine which set of units to use by default

(Sidenote: After playing around in Ampersand for the last month, Backbone feels really outdated.)