Add reverse() method to geocode module

Take a bunch of points and turn them into addresses.

Addresses #111.

This turned out to be pretty easy as the the forward and reverse geocode functions return the same thing.
Possible future enhancement would be to allow coordinate tuples instead of Shapely `Point` objects.

It would be nice if the reverse geocoding split out administrative levels, but it doesn't look like most of them do it now.
Somehow missed this until now. Sounds good.

I think importing the geocoding stuff into a `tools` module sounds like a great idea. Maybe leave the code separate though..?