add new binary protocol operation for Observe

To facilitate several use cases relating to applications wishing more information on durability and whether or not data will be considered in stale=false view requests, we should add an Observe command.

The description of this command is covered here:
http://www.couchbase.com/wiki/display/couchbase/Observe

Note that previous attempts at using stats key did not work out because of the protocol required ASCII representation of results in stats key.