[mapistore-python][sample-backend]: enable get and set properties for attachments
[mapistore-python][sample-backend]: fix set_properties and save (changes in folders and messages didn't have any effect)
[mapistore-python][sample-backend]: implement delete_message (hard deletion)
[pymapistore]: Changes in get_properties behaviour

* If there are no arguments, returns all the properties
* If the argument is a list of properties, return these properties
* If the argument is an empty list, return an empty dictionary
pymapistore: Raise an error if the data returned from get_properties is an invalid pointer