Update for geopy 0.96 compatibility

`geopy` 0.96 [removed support](https://github.com/geopy/geopy/blob/master/RELEASES) for the Google Maps v2 API, which removed the `geopy.geocoders.Google` attribute and broke things, causing tests to fail.  The Yahoo API has also changed to [`YahooPlaceFinder`](http://geopy.readthedocs.org/en/latest/#geopy.geocoders.YahooPlaceFinder).

Also added support for the OpenStreetMaps [Nominatim](http://geopy.readthedocs.org/en/latest/#geopy.geocoders.Nominatim) geocoder which is now in `geopy`.  To honor their [usage policy](https://wiki.openstreetmap.org/wiki/Nominatim_usage_policy), requests are throttled to 1 request per second.

Added a few explicit geocoder tests as well (these require network access, of course).  There are still no tests for APIs that require a key.