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. Then, you can run the `rake locations` task to easily populate coordinates for each location. The resulting data will be saved in the format the map expects.

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 without changing the look of the existing site.

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/
Could we make this task name a bit more explicit, mapping it as an activity, rather than a noun. 
```rake locations``` requires more context than something like ```rake generate_json_for_locations_map```