Add support for reverse geocoding

Add methods to Geocodio::Client to support their new reverse geocoding
API endpoints. To reverse geocode, pass a coordinate pair in the form of
a string or a hash to Geocodio::Client#reverse_geocode:

```ruby
geocodio.reverse_geocode('37.331669,-122.03074') # Or...
geocodio.reverse_geocode(lat: 37.331669, lng: -122.03074) # Or...
geocodio.reverse_geocode(latitude: 37.331669, longitude: -122.03074)
# => #<Geocodio::AddressSet @addresses=[...]>
```

Signed-off-by: David Celis <me@davidcel.is>
DRY up Geocodio::Client

Extract similar fragments of code responsible for parsing response
bodies into a Geocodio::Utils class.

Signed-off-by: David Celis <me@davidcel.is>
Create new VCR cassettes for reverse geocoding changes

Signed-off-by: David Celis <me@davidcel.is>