storage.bucket.Bucket: add direct support for additional resource fields

[tseaver]

The Bucket API resource representation defines a number of fields as "writable", but which we don't already map:

• cors[]
• lifecycle{}
• location
• logging{}
• versioning{}

The support for the website{} field is slightly odd (Bucket.configure_website / Bucket.disable_website), and should likely be normalized with support for the other fields.

Bucket.from_dict currently captures them all in the key's metadata attribute, but that name creates confusion with its metadata key (which stores application-defined key-value pairs).

I think it would be cleaner to map each the non-user-defined fields onto separate attributes, and propagate them to the server via the insert, and patch or update endpoints.

Bucket should also expose read-only properties for the non-writable resource fields not already mapped, and which aren't constants:

• etag
• id
• metageneration
• name
• owner{}
• projectNumber
• selfLink
• storageClass
• timeCreated

[tseaver]

Note that support for lifecycle{} allows for a natural solution to #162: buckets with no lifecycle{} defined just overwrite on uploads.

[tseaver]

See the lifecycle spec for notes on configuring a lifecycle policy.

[tseaver]

Closed via #347.