Add 'exclude_from_indexes' method to Entity.

Set it via ctor argument.

Pass it to 'Connection.save_entity'.

Fields in the sequence will have the 'indexed' field set False in the
corrsponding protobuf.

Fixes #83.
Expose 'exclude_from_indexes' via 'Dataset.entity()'.

Incorporates feedback from @dhermes.
Avoid mandating type of field names passed to 'exclude_from_indexes'.

Incorporates feedback from @dhermes.
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 #330 from tseaver/83-expose_entity_indexed

Fix #83:  Add 'exclude_from_indexes' method to Entity.
Merge pull request #333 from dhermes/refactor-key-bucket-shared-code

Refactoring duplicate code between storage.key and storage.bucket.
Adding extra parameters for querying Cloud Storage keys.
Testing interaction with page token and extra params in iterator.

Also making sure tests pass by using the `upload_` prefix
instead of the `set_contents_` prefix.