implemented simple go library to check creds via saslauthd

This gives us ldap auth integration

Change-Id: Ic0b58d1cd9136f0bd39b8d800222a0b9ce5ae751
Reviewed-on: http://review.couchbase.org/43973
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>