Add API Key setting for Google Maps

Google limits the number of requests an application can send without an API key. These limits are raised significantly if a key is included in the request. This patch adds a new setting to the settings form in which you can include your key. In the MapProviderGoogle class, the buildParams() function will add the key to the request if one exists. See a live example here: https://www.nantucketchronicle.com/nantucket-local-favorites/getting-around