Add enterprise-only module: cbauditd

Change-Id: I504b6ee10f91211a9f98fcd3e7a461b23579d9cf
Reviewed-on: http://review.couchbase.org/43356
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>