New feature: allow Google Cloud Storage S3

[pierre-b]

Hi,

The S3 compatible Google Cloud Storage endpoint is not handled properly, I get error:

error testing destination: failed to test S3 bucket: operation error S3: HeadBucket, https response error StatusCode: 403, RequestID: , HostID: , api error Forbidden: Forbidden

I've tested the same bucket/endpoint with other S3 compatible projects successfully.

It's probably in the way the endpoint url is constructed at the end by the S3 sdk (https://storage.googleapis.com/BUCKET_NAME)

[pierre-b]

Would be great to merge PR #141 in case it fixes the same issue please!