Add Page implementation for testing

[mderka]

Add Page a simple PageImpl under https://github.com/GoogleCloudPlatform/gcloud-java/tree/master/gcloud-java-core/src/main/java/com/google/gcloud/testing that gets one or more list of values.

[garrettjonesgoogle]

It's not clear what the need is. This seems like something that can just be done when it's actually needed. Closing this out.