Disambiguate s3 and gs anonymous auth

Initially reported in [this StackOverflow post](http://stackoverflow.com/questions/32626259/gsutil-to-copy-anonymously-from-s3).

Presently, [ConfigureNoOpAuthIfNeeded](https://github.com/GoogleCloudPlatform/gsutil/blob/40fc7c0843513ab719e5061b7b3547b07605fb8b/gslib/util.py#L347) is only called when no credentials are available for s3 or GCS. However, it is possible that a user may want to access one anonymously while using their credentials for the other.

We should have separate no-op auth handlers for both s3 and gs capabilities so that they can be used when necessary.

This will likely require refactoring of the auth helper logic in gslib/util.py, as we want to avoid boto.auth.get_auth_handler connecting to a service endpoint unnecessarily.