Support saving notebooks to Google Cloud Storage

I tried using the new HCFS storage implementation (FilesystemNotebookRepo) with the GCS connector for Hadoop (https://github.com/GoogleCloudPlatform/bigdata-interop/tree/master/gcs), but the UI was a little slower than I would have liked. I assume this is due to extra roundtrips for HCFS directory semantics, which are largely unnecessary for Zeppelin storage.

I wrote a GCS storage implementation called GCSNotebookRepo (will make a github PR), similar to the ones for S3 and Azure.