CardDAV address book support

I just pushed the new branch carddav-ab which implements limited support for remote CardDAV address books in Balsa. Please see the file README-CardDAV.md for details about the implementation and its limitations – as CardDAV address books usually contain a plethora of information in its VCARD elements, modifying them using Balsa's the rather limited address book editor would probably break them. Thus, it is only possible to read the address book and add items, but modifying or deleting elements is not supported (use e.g. your mobile phone for that…).

I divided the implementation into a WebDAV and a CardDAV layer. Rationale: Iff we decide to include it in Balsa, we might also want to have CalDAV support in the future (to add iCalendar requests to a remote calendar), and splitting these logical layers would simplify the implementation.

It would be great if someone could have a look at it, before I create a merge request. As always, any comment would be highly appreciated!

Thanks, Albrecht.