* Add 'google_billing_account' data source.
* Use 'GetResourceNameFromSelfLink'.
* Use 'ConflictsWith' in schema.
* Use pagination for List() API call.
* Add ability to filter by 'open' attribute.
* Don't use 'ForceNew' for data sources.
* Add 'billing_account' argument and make 'name' an output-only attribute.
* Correct error message.
