More than one result is very rare and it's not clear how one would
evaluate the choices. If this is required in the future a new method
can be added.

Don't take 'reverse' argument, instead figure it out by looking at what
options were specified for the model. Also, don't raise exceptions on
bad configuration (no longer possible to detect, wasn't that important).

city, country, country_code, postal_code

For advanced/custom handling of gecoding result data.

Also move 'address' and 'coordinates' methods from Geocoder::Lookup::Base
to Geocoder. This simplifies the interface and allows us to choose a
lookup engine dynamically for other types of geocoding (eg: IP addresses).

Add implementation for Google and anticipate addition of Yahoo (supply
config option).

This allows the same model to be forward and reverse geocoded and still
provide a geocodable address though a non-attribute method. Also raise
an exception if sufficient configuration is not provided.

Move some methods into it (organize!).

Should be slightly faster and we avoid an ugly REXML require statement.

Was using version 2. Version 3 no longer requires an API key so this is a big
improvement to the getting started experience.