Add a Google Cloud Storage filesystem/target.

In theory, GCS is supported by boto, which would be a better option. Unfortunately Google's API authentication is a
mess - the boto GCS implementation basically only supports the CLI use-case (it expects a boto config to house all
the configuration). It has minimal support for using oauth tokens, but there is no way to use a service account,
which is what they recommend you use for automated tasks. You can see it at https://github.com/GoogleCloudPlatform/gcs-oauth2-boto-plugin .

Because of this insanity we use the google-api-python-client package to speak to GCS via their JSON protocol. This
is...not easy. Either way, this change does work and passes quite a few tests. Unfortunately since the API is
auto-generated, you can be sure that there are no fake implementations of it (e.g. moto). Hence the test is an
integration test and requires credentials on your box to access GCS.

You can get creds by going through the gcloud tools setup guide. After that just create a project & pick a bucket
and everything should work :)