Replace BucketInfo.StorageClass and BucketInfo.Location classes with plain strings.

[aozarov]

Issue was raised by @mfschwartz and @thobrla which find the enum as a reference for the current
set of values confusing.

When replaced to strings we should add a link in the Javadoc of the relevant BucketInfo.Builder setter to a page with the list of values.

For location we can use - https://cloud.google.com/storage/docs/bucket-locations

For Storage class we can use - https://cloud.google.com/storage/docs/storage-classes
(and/or https://cloud.google.com/storage/docs/json_api/v1/buckets#resource as unfortunatly
the documentation of possible values is not as good as location - @mfschwartz is it possible to fix that?)

[aozarov]

@mziccard do you want to take that as well?