pymapistore: Raise an error if the data returned from get_properties is an invalid pointer
[pymapistore]: clean up the commented code in the Python bindings
Merge pull request #117 from icandigitbaby/jkerihuel/mapistore-python

Clean up commented code in the MAPIStore Python Bindings
[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
[mapistore-python][sample-backend]: fix attachment ID behaviour, use attachment cache and add set_properties
[mapistore-python][sample.py]: include "next_aid" in message entries
[mapistore-python][sample-backend]: implement create_attachment
[mapistore-python][sample.py]: fix white spaces and line breaks