Subdivisions

Added support for handling a single subdivision and subdivision code:

```ruby
irb> look_up "20.20.20.20" # subdivision known
  => {:city=>"Falls Church", :subdivision=>"Virginia", :country=>"United States", :latitude=>38.864, :continent=>"North America", :country_code=>"US", :longitude=>-77.1922, :subdivision_code=>"VA"}

irb> look_up "4.5.6.7" # no subdivision known
  => {:city=>nil, :subdivision=>nil, :country=>"United States", :latitude=>38.0, :continent=>"North America", :country_code=>"US", :longitude=>-97.0, :subdivision_code=>nil}
```