Use tipsy for post scope tooltip

app/views/shared/_stream_element.html.haml

@@ -2,6 +2,11 @@
 -#   licensed under the Affero General Public License version 3 or later.  See
 -#   the COPYRIGHT file.
 
+:javascript
+  $(function() {
+    $("span.post_scope").tipsy({trigger: 'hover', gravity: 'n'});
+  });
+
 .stream_element{:id => post.guid}
   - if user_signed_in?
     - if  post.author.owner_id == current_user.id