Upstream and finish `Blob#compose` for GCP

[julik]

We have a set of bits for this internally but it hasn't been ported into the gem yet

[porterbayne]

I'm interested in using this gem. We do use Google Cloud for our app, including storage. Does this issue indicate a potential blocker?

[julik]

@porterbayne I am no longer with Cheddar, your best bet is @svanhesteren

[julik]

You can take a look at #10 and maybe pick it up and finalize it? there ain't much there. GCP works fine if I remember correctly - just the blob compose is not working because we had to implement our own resumable upload for GCP, and to test it you want a "real" GCP bucket, the credentials... the works