Use getters for DatastoreOptions and the Datastore service (GoogleCloudPlatform#401)

`defaultInstance()` and `service()` seem to be deprecated in favor of `getDefaultInstance()` and `getService()`

Author: glaforge

datastore/cloud-client/src/main/java/com/example/datastore/QuickstartSample.java

@@ -26,7 +26,7 @@
 public class QuickstartSample {
  public static void main(String... args) throws Exception {
  // Instantiates a client
- Datastore datastore = DatastoreOptions.defaultInstance().service();
+ Datastore datastore = DatastoreOptions.getDefaultInstance().getService();
 
  // The kind for the new entity
  String kind = "Task";