Sort should accept a list of pairs (tuples) as arg

[pauloalem]

Dictionary order is not guaranteed and the driver should accept a list a of pairs (tuples) as specified in http://api.mongodb.org/python/current/api/pymongo/cursor.html#pymongo.cursor.Cursor.sort

[tahajahangir]

Client.sort already accepts a list of pairs (tuples) as sort argument (related to #35 ).

Really, the value passed as sort argument is not checked to be a list of pairs, it is passed directly in $orderby key to mongodb, so it can/should be a list of pairs.