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.