Add get_logging_config to GCS bucket

This adds get_logging_config functionality to gs buckets and storage_uri, to
retrieve the logging configuration of a Google Cloud Storage bucket. Also makes
a number of smaller fixes for consistency.

Note that this breaks the getwebcfg command in gsutil. A change to address that
is that being submitted concurrently.

Code reviewed under https://codereview.appspot.com/7381057/