Adds methods for calculating the bearing between two points and for converting that bearing into a compass point, e.g. North or South West. If you want fewer or additional compass points (e.g. South By South West, or just NESW) provide a second parameter containing your array of points, e.g. Geocoder::Calculations::compass_point(90,['north','east','south','west'])

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.

Now that we also support an API key for Google it makes sense for this to
be generic.

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).