Install extension/engine dll to lib/memcached

Change-Id: I9465c4398bc417d89431c7f4c6833c426715c7c1
Reviewed-on: http://review.couchbase.org/35135
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>