title: cloud.google.com/go/pubsub iterator.go:44: undefined: acker and puller
body: <p>I'm hitting the following error messages when I build codes that employ cloud.google.com/go/pubsub.
  Am I working on a wrong package? Or it's just because of alpha?</p>
  
  <pre><code>$ go test
  # cloud.google.com/go/pubsub
  ./iterator.go:44: undefined: acker
  ./iterator.go:46: undefined: puller
  FAIL    cloud.google.com/go/pubsub [build failed]
  </code></pre>