added _cbauth endpoint

Change-Id: I3f620aab8d3f1203b4a5a78cb71802b3de9bfef3
Reviewed-on: http://review.couchbase.org/43658
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>