make the carddav backend work with android and baikal/sabredav

Hi,

Here is a patch to make the carddav backend work with android and baikal/sabredav.

I couldn't get it work with my carddav server based on sabredav and I actually don't know how it could have been working because neither sabredav nor davical seem to suport the "sync-collection" option on carddav.

So I made this patch that try to search all the vcards within a defined addressbook.

Hope you'll enjoy it