Calculate geoip accuracies from country bounding box data, closes #145.

@rtilder, @graydon can one of you review and potentially merge this?

This should take care of returning a country specific accuracy value, continue to return a smaller city accuracy value, but also take into account small countries and return for example 1km accuracy for the Vatican.

I've opted for low-level unit tests for most of this, as we don't have a country-only entry in our test GeoIP.dat file.