Embed root CA certificates to follow the update of the server.

* twittering-mode.el: Embed root CA certificates to follow the
update of the server. The all root CA certificate available at
https://www.symantec.com/page.jsp?id=roots are embedded.
(twittering-ca-cert-list): add root CA certificates.
* cert/collect.sh: Add a script to collect and integrate root CA
certificates.