Refactoring duplicate code between storage.key and storage.bucket.
Fixing lint error in storage._helpers.
Removing ACLMetadataMixin and unused ACL_CLASS.

Also removing make_public() from base mixin class.
Per review feedback from @tseaver.
Merge pull request #333 from dhermes/refactor-key-bucket-shared-code

Refactoring duplicate code between storage.key and storage.bucket.