Implement publishBatch and pullBatch methods

API specs:

Ideally, publishBatch could accept messages in a raw or simple format.

According to #377 (comment) the suggested way of publishing and pulling messages is now through these methods on top of our existing publish and pull methods. We can just rework publish to support an array of messages and always use publishBatch API call behind the scenes. Similarly, pull right now returns 1 message so we can either use maxEvents: 1 by default to maintain backward compatibility or we can pull many messages, and document the new maxEvents option. Thoughts?

Just accepting an array to publish is a great idea. Same for pull; just accept an object with maxCount (instead of maxEvents?) that if absent, will default to 1/current behavior. 👍

I'd like to promote the priority of this one, since it breaks every build (which is now 3 builds to account for each supported platform) - https://travis-ci.org/GoogleCloudPlatform/gcloud-node/jobs/50067298