Implement OBSERVE

Change-Id: I63e3d5edeed3874697b5d4a230e5542ae515a1d7
Reviewed-on: http://review.couchbase.org/28614
Reviewed-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>