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 dont think we need such an event.

topic.subscribe({ name: 'whatever', ackDeadlineSeconds: 80  });
- should get subscription called whatever and modify ackDeadlineSeconds is one is set
- if doesn't exist, it should create it
- if exists, should modify ackDeadlineSeconds
- emit ready event