Prefer to rely on lower level methods (reduce dependencies).

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).

Prepare to add support for Yahoo in addition to Google.

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

Documents Geocoder.search method.

Except ZERO_RESULTS, which doesn't seem to merit a warning.

Wrong number of arguments for reverse geocode search.

Move to fetch_parsed_response. Will use search method for fetching a
Result object.