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.
> I didn't think it was, but it was the only way to update a property of a subscription that I could find.

PubSub API authors say typical usage is to create topics in advance. I don't think we should add complexity to code base to support a behaviour that is not expected neither works great.