Implement MaxData GeoIP2 provider and database adapter

Update README.md, add requirements/suggest to composer.json.
Change access of GeoIP2 database-reader, due to problems with JSON-encoded geodata.
Refactor GeoIP2Provider: Use injection of Maxmind provider.

Instead of using only the database reader of Maxmind, inject the Maxmind provider. Thus both is available: database reader and webservice client.
Merge pull request #303 from jenswiese/geoip2

Implement MaxData GeoIP2 provider and database adapter