Add support for a Google Maps API key

From https://developers.google.com/maps/documentation/geocoding/#api_key:
All Geocoding API applications should use an API key. Including a key in your request:
- Allows you to monitor your application's API usage in the APIs Console.
- Enables per-key instead of per-IP-address quota limits.
- Ensures that Google can contact you about your application if necessary.
fix cs
update google maps provider doc
Merge pull request #306 from toin0u/hotfix-googlemaps

Support Google Maps API key