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.
This function name is too generic for my taste.  I'd prefer `reverse_geocode`.

Related: The new `sjoin` and `overlay` functions will go into a `tools` module.  Should `geocode` stay a separate module, or join them in tools?