Add a tag to notifications so they can clobber. Fixes #159.

This is the same fix we applied to angular to fix this problem in SYWEB-21.

src/skins/vector/views/organisms/Notifier.js

@@ -66,7 +66,8 @@ var NotifierView = {
             title,
             {
                 "body": msg,
-                "icon": avatarUrl
+                "icon": avatarUrl,
+                "tag": "vector"
             }
         );