Implement the CardDAV Directory Gateway Extension for Apple clients (and SOGo connector).

This exposes an LDAP-based address book inside the principals address book collection.
According to the spec [1], this should be used for querying only. Listing, however, can
be enabled by setting 'searchonly' => false in config.

CARDDAV:addressbook-query requests will be translated into LDAP filters
and post-processed by SabreDAV internals.

[1] http://tools.ietf.org/html/draft-daboo-carddav-directory-gateway-02