For consistency with rest of code.

This has always been the intended behavior (as shown by the README's
sample code which begins `if results.first`) but it wasn't implemented
properly. Thanks to github.com/nhippenmeyer for pointing it out.

Removed set_defaults from all tests and replaced with teardown to reset the defaults.

Signed-off-by: James McCarthy <james2mccarthy@gmail.com>

Allows custom handling of geocoding service connection problems.

Fixes #56

After further consideration, multiple results should be the default
behavior for this method. Some services only return one result (eg: any
IP-based service) but a great feature of advanced services like Yahoo
and Google is that they return an array of choices when doing a vague
search like "100 Main St" with no city, region, or country specified.

Google results don't return a 'city', so we have to look at several
fields in descending order of preference.

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).

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.

No longer necessary.

For advanced/custom handling of gecoding result data.

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

This should have been part of commit 70cc64ee.

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 all network/lookup-related methods into it.