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/
This will happen as soon as the Rakefile is loaded, I think. This means that all calls to any rake task will cause this to happen, right? Could lead to unexpected behavior in the future, if someone adds a rake task. Could we make a rake task like ```initialize_geocoder``` which the generate locations task is dependent on? So, it will get executed explicitly before any tasks that require it. Or at least make it a helper method called at the beginning of the tasks that need it.