Add functionality to sample backend

* set_properties
* save (message)
* delete_message
* create_attachments
* get_attachment_ids
List comprehensions are intented for those cases where you want to do something with the resulting list, in this case, I'd use:

```
for msg in self.folder.basedict['messages']:
    if msg['mid'] == self.mid:
        self.folder.basedict['messages'].remove(msg)
        # should we break here? taking into account the lenght of the list has changed...
```