The Dconf Editor
Edit the various properties in gnome shell.To install
yum install dconf-editor
To get the Seconds and date in the clock Start the dconf editor and Change the values in org>gnome>shell>clock.
Also you can tweak other settings.
Then Press Alt+ F2 (with Function Key )
and type dconf-editor
There is a fantastic developer called fpmurphy developed lot of great gnome extensions You can read his blog for development of custom extensions.
There is another fantastic blog called Webupd8 They are collecting all type of gnome shell extensions.
