Support versioning more and parsing objects

• Fix url encoding issue for multipart uploads (#139 - thanks @jomatt)
• Add downloading a folder example to gcs_get_object() #144
• Improve message feedback for downloading objects
• gcs_upload() will now retry with a warning if using legacy ACL settings for a bucket with uniform bucket-level access enabled - will retry with predefinedAcl='bucketLevel'
• Add gcs_parse_rds() to help parsing RDS objects for gcs_get_object() #146
• Add ability to request non-current versions of objects in gcs_get_object() via generation parameter.
• Add gcs_delete_bucket_objects() to help delete all objects from a bucket, including older object generation versions (#152)