added NewAuthHandler to integrate cbauth with go-couchbase

Change-Id: Iee0c6568255efcf09864f0a4b1d618229a6e02aa
Reviewed-on: http://review.couchbase.org/44548
Reviewed-by: Manik Taneja <manik@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>