pubsub: refactor (fixes #98).

Here's the PubSub API refactor (#98)! The [readme updates](https://github.com/stephenplusplus/gcloud-node/blob/pubsub/README.md#google-cloud-pubsub) + tests ([regression](https://github.com/stephenplusplus/gcloud-node/blob/pubsub/regression/pubsub.js) & [unit](https://github.com/stephenplusplus/gcloud-node/tree/pubsub/test/pubsub)) will be good places to see it in action. Let me know if you see anything you think can be improved!

Note: this could likely still use a few more tests, and definitely needs a sweep for doc block accuracy. I'll get to these soon.
Added in an `autoPull` property for cases like these: a user is fetching all the subscriptions. They probably don't intend to start pulling from each one automatically. `autoPull` is true by default, but false in this case. They will have to manually call `sub.startPulling()` to begin the pulling.