71bd967b43
Now that the handling for multiple address books is in place, this commit adds initial support for creation and deletion of address books. These operations obviously require support from the backend, so the interface gains two new methods. All properties of the address book passed to `CreateAddressBook()` may be unset (e.g. when a client sends a MKCOL request without a body), except for the path, which is always set. It is up to the backend to put any desired default values in place.
