PHP-Push-2: Updated to Z-Push 2.1.1 and modified Caldav Backend

Hi dupondje,

first of all - this is my first pull request ever. So please excuse me if something's not quite as it should be. 
I stumbled upon PHP-Push-2 when I tried to get a Baikal server to work with Z-Push. It turned out that almost everything went really smooth (thanks!), apart from the CalDav backend. For my server setup, I couldn't get authentication to work. After some research I decided to re-write the caldav-client-v2.php using ideas from carddav.php.
I hope that my modifications are useful for other people, too. Since your repository is as "upstream" as I could find, I trust that you are the maintainer of this package. If this is so, I would be glad if you could consider my pull request for inclusion. If you're not the person responsible, please point me in the right direction. Many thanks in advance!

Best,
tkoester