Maintainable locations

I want to make it as easy as possible to add locations to the map, so this commit lets you specify the city names in the `data/locations.yml` file. When we deploy to Heroku, a rake task will process each location, using the [Geocoder] gem to find the coordinates and storing the result in the correct format.

This PR is independent from #4. If you want, you can merge it, edit the `data/locations.yml` file, and redeploy to fill in the missing cities.

Let me know if there's anything about this process you'd like changed. This is just a first pass at solving the problem.

[Geocoder]: http://www.rubygeocoder.com/