add constantTimeDispatcher and use it for strategic refreshes.

constantTimeDispatcher lets you poke a specific react component to do something
without having to do any O(N) operations.  This is useful if you have thousands
of RoomTiles in a RoomSubList and want to just tell one of them to update,
without either having to do a full comparison of this.props.list or have each
and every RoomTile subscribe to a generic event from flux or node's eventemitter

*UNTESTED*

src/components/structures/TimelinePanel.js

@@ -523,6 +523,9 @@ var TimelinePanel = React.createClass({
                 this.props.timelineSet.room.setUnreadNotificationCount('highlight', 0);
                 dis.dispatch({
                     action: 'on_room_read',
+                    payload: {
+                        room: this.props.timelineSet.room
+                    }
                 });
             }
         }