Implements Draft RFC CardDAV Directory Gateway Extension

See https://tools.ietf.org/html/draft-daboo-carddav-directory-gateway-02

Signed-off-by: Maël Lavault <mlavault@alinto.eu>
Merge pull request #26 from moimael/master

Implements Draft RFC CardDAV Directory Gateway Extension

Need to move the getDirectoryGateway to an extension interface