NCBC-542: Provide Observe method on IBucket

Change-Id: Iecfbc02490b9ce237d6fbe16619e5aa252566bd7
Reviewed-on: http://review.couchbase.org/40351
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>