Added basic digest authentication support to CalDAV client

I've been working to get PHP-Push-2 working on a recent OS X Server install (using 10.8.2 Mountain Lion), and for whatever reason OS X Server requires digest authentication to work.

This patch adds the most basic version of digest authentication to the caldav-client-v2.php file as a fallback if basic authentication fails (and there is a `WWW-Authenticate: digest` header in the response from the server).