added method ensure_objectid in db_utils to convert strings or unicode to bson ObjectId's if they aren't already.

added new method merge_update_args to db_utils for merging an array of arbitrary updates to a single update in mongo.

Updated property keyword replacer to handle cross references (works with arbitrarily nested cross references).

simplified logic and updated mongo_insert_one and mongo_insert_many to return the id/ids of the newly inserted records.

changed the default behavior of mongo_find_records to not serialize to named tuple, since i was never using this and tired of specifying keyword everytime