[mapistore-python][REST]: Correct trailing commas in kissDB
[mapistore-python][REST]: Correct `_encode_restrictions` (naming & return value)
[mapistore-python][REST]: Add description of the parameters in kissDB.py
[mapistore-python][REST]: Implement `messages_id_exists` in api_handler.py
[mapistore-python][REST]: Add attachment POST operation
[mapistore-python][REST]: Add attachment PUT operation
[mapistore-python][REST]: Add attachment GET operations (get attachment from ID & get attachments in a message)
[mapistore-python][REST]: Add attachment GET operations (get attachment from ID & get attachments in a message)
[mapistore-python][REST]: Add attachment DELETE operation
[mapistore-python][REST]: Implement create_attachment + attachment class
[mapistore-python][REST]: Implement attachment save