Remove backward-compatibility aliases.
Docstring fixes.
Drop (unused-and-confusing) '_has_property' method.
Use 'properties' to ensure loading before lookup.
Fix copy-paste errors in 'Bucket' accessor docstrings.
Avoid remapping names of keys in server-side properties.

Per @dhermes, https://github.com/GoogleCloudPlatform/gcloud-python/pull/347#discussion_r19950864.