Remove messages from notifications when read

Remove individual messages from Notifications when marked read.
Previously this was only done from the conversation model when marking
the entire conversation as read.

Fixes #717

// FREEBIE

js/models/conversations.js

@@ -403,7 +403,8 @@
                     message        : message.getNotificationText(),
                     iconUrl        : iconUrl,
                     imageUrl       : message.getImageUrl(),
-                    conversationId : conversationId
+                    conversationId : conversationId,
+                    messageId      : message.id
                 });
             });
         });