Add GeoLite2 support, both City and Country databases
Add tests and make them pass
Add description of how to use GeoLite2 for IP geocoding
Add support for pure Ruby access to MaxMind database through maxminddb gem
Merge branch 'feature/geolite2_support' of github.com:ChristianHoj/geocoder into pr/732
Rename from :geolite2 to :geoip2.

GeoLite2 is a free version of a product called GeoIP2. Both use the same
format.
Remove "EXPERIMENTAL" from GeoLite2 API.
Clarify README.
Rename config setting from :maxminddb_gem to :lib.