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.
Since sometimes we create a Topic object in the callback from a "GET /topics" request, those return back to use the full "/topics/project-id/topicname" format. This normalizes all "name" input to "topicname" in that case - specifically, the line removes all characters up until the last '/'. The full name is then constructed in the format expected by the api.