Integrate Google Cloud Storage

RE: #333 

Thought I'd start with a PR right away.

I'm still making my way through the codebase. I have a test file that uses my credentials, and the pkgcloud storage API works as expected (I think).

Unfortunately, I'm still not sure how to properly integrate with the tests properly.

[This guide](https://github.com/GoogleCloudPlatform/gcloud-node/#elsewhere) shows how you can create a project & download a JSON key. That should be all you need to run the test file (+ setting the mock environment variable).

// @silvolu @erjohnso 
So, I see that you already support creating the writableStream with gcloud native, so we don't have to make a follow-on call explicitly to get the model, so we're just fixing up the stream events. Looks good.

One benefit to using the common storage tests is to ensure consistency across providers with the stream api :)