Implement GCS storage; resolves #1

Build and run:

    go build && GCS_ISS=? GCS_KEY=? PIXLSERV_GCS_BUCKET=? ./pixlserv run config/example.yaml

I have omitted the values of the environment variables above. 

I have tested this against a test Google Cloud Storage bucket, and it works. I can send you the credentials (privately; may be email, or direct message via Twitter?) of the test bucket I created if you want to test it on your end, as the process is a bit tedious, imo. 

To test, all you have to is run the above command with the environment variable values in place.

For Google Cloud Storage, `GCS_ISS` is something that looks like an email address, a pretty long one actually! And `GCS_KEY` is a private key (approx 1.1KB in size).