Implement OBSERVE

Change-Id: Ib2823c9ce74a55ff38f193cb03d2e5fd510df70e
Reviewed-on: http://review.couchbase.org/27713
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>