Modified Caldav Backend

Hi fmbiete,

dupondje pointed me your way when I tried to contribute to his PHP-Push-2 yesterday.
I had to modify the caldav backend because I couldn't get it to work with my server setup. The home-brew mechanism in caldav-client-v2.php would get the authentication wrong, so I switched to cURL.
I hope that these modifications are useful for other people, too, and I would be glad if you could merge them with your codebase.

Best,
tkoester