Add functionality to sample backend

* set_properties
* save (message)
* delete_message
* create_attachments
* get_attachment_ids
Either add a check here:

    if (attachment_object) *attachment_object = pyattach; 

or add a check at the beginning:

    MAPISTORE_RETVAL_IF(!attachment_object, MAPISTORE_ERR_INVALID_PARAMETER, NULL);