Replaces read_coordinates (private) and fixes bug in geographic center
computation (reported by github.com/jonbuda).

This was causing an exception (thanks to github.com/mikepinde).

Does not seem like a significant gain to have the auto-assignment AND a
custom block. In fact, it seems annoying to have to use a different
method. So now, passing a block to geocoded_by or reverse_geocoded_by
prevents any auto-assignment from happening.

Add geocode and reverse_geocode methods along with bang versions
which can be used to skip auto-assignment of address or coordinates
(specify a block that handles the Geocoder::Result instead).

Be sure to return nil.

If forward and reverse geocoding were both declared and options passed
for both, only the options for the first were being set.

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.