Add support for MaxMind Database ( GeoIP City database )

This pull request adds support for MaxMind Database ( free and paid ).  It's really nice since it doesn't depend on network and a remote server.

A few points that the gem's author should consider:

1) It adds a new requirement. The geoip gem.

2) In order to test it i had to add a copy of the free GeoLiteCity database. It will increase the repository size ( 19MB ) and you might want to check if it's compatible with gem's license.

3) Some tests started to fail since they seem to consider that the lookup class will have an API Key, which is not our case here. Eg.: https://github.com/fernandomm/geocoder/blob/master/test/result_test.rb#L9

I had to exclude this lookup class from these tests https://github.com/fernandomm/geocoder/commit/da90c6c81458b1b9b1b178e4dac100012e8a7eb9

I would appreciate some feedback about these points :+1: