ZP-299 #comment moved imap, vcard and maildir backend into an own directory. Moved backend configuration options from main config.php to the backend config.php files. Implemented backend autoload (order: zarafa, combined, imap, vcard, maildir) if no backend provider is defined. Fixed warnings of combined backend (needs testing!) #time 1h 30m

git-svn-id: https://z-push.org/svn/z-push/trunk@1594 b7dd7b3b-3a3c-0410-9da9-bee62a6cc5b5