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.
> Don't delete subscriptions, that's not the intended behaviour. There may be multiple programs running that is using this subscription you're using. Deletion will break them all.

I didn't think it was, but it was the only way to update a property of a subscription that I could find. If you'd still like this behavior (with whatever overall API we end up with), we'll need to figure this one out.