PYCBC-107: Support dict_keys

Change-Id: Id1968d3e536ffb00603c40792c136f6c8250994f
Reviewed-on: http://review.couchbase.org/26276
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>