Initial support for Google Cloud Storage

This is an initial support [Google Cloud Storage (GCS) JSON API](https://developers.google.com/storage/docs/json_api/).

To simplify the review process, this PR contains only the initial scaffolding of the GCS provider and some domain objects related to the buckets (with unit tests).

As I don't have enough rights on this repo, I can't explicitly assign reviewers.
From IRC I know @mattstep @andrewgaul @adriancole should be interested on looking into it. Thanks!