Support access via IAM credentials

Simply, the documentation in the Readme states that this library requires 'A Google Maps API key.', but most other services I've used (like Storage, for example) allow and default to using IAM credentials supplied via environment variable GOOGLE_APPLICATION_CREDENTIALS. When enabling the Places API through console, it lists 3 forms of valid authorization - API key, OAuth (for accessing user data), and IAM. Since the console supports IAM access, shouldn't this library?

So the request is either to support this, or to update the readme if it is just supported somehow.