Add support for queries with earth_distance

As suggested in issue #2, I think it would be useful to access to the more accurate `earth_distance` test rather than just the `earth_box` check. 

I think the ideal situation is to combine the two to do a cheaper `earth_box` check followed by a more accurate `earth_distance` check if the former is successful. I don't need that quite yet so I have included the lazier approach of using the `earth_distance` check directly. More details in the commit message.

I hope it seems reasonable, though I think the naming could be shuffled around a bit. Thanks for writing the app.

Michael