Add option to set a local IP substitute

I'm using this feature which is quite handy in development mode. Basically it allows your app to have sensible data while running on your local (development) machine, which is handy to test out all the features.

```ruby
$geoip.city("127.0.0.1")
# => nil

$geoip.local_ip = "173.194.112.35"

$geoip.city("127.0.0.1") 
# => #<struct GeoIP::City request="localhost", ip="173.194.112.35", country_code2="US", country_code3="USA", country_name="United States", continent_code="NA", region_name="CA", city_name="Mountain View", postal_code="94043", latitude=37.41919999999999, longitude=-122.0574, dma_code=807, area_code=650, timezone="America/Los_Angeles">
```