Add a Turba_Driver::canAdd() method for indicating the driver's ability

Add a Turba_Driver::canAdd() method for indicating the driver's ability
to add a new contact.

Needed because we don't have perms on individual contacts, just on the
contact's address book. So for sources such as a virtual address book,
we may be able to edit the contact, but we can't add new contacts to
the address book.