PYCBC-36: Implement OBSERVE command

This wraps the low-level OBSERVE command. This may be used to implement
durability/persistence requirements, or simply observe key storage
status

Change-Id: I1409f8e6ce6c87cf0b1176bf77840eb603025583
Reviewed-on: http://review.couchbase.org/26734
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>