CardDAV support for owncloud / sabredav plus external IMAP provider

Based on the CardDAV code of ziirish there is an working CardDAV backend for owncloud / SabreDAV. 
(see issue #80 / #76 / #47/  #71  )

to support my szenario there is a need to support external email provider with complete email address as loginname
Owncloud, can not handle Email adresses as usernamens. To use PHP-Push-2 in this requirement I've added some code