Retain unknown VCard properties, ez-vcard update, handle stale connections

* store unknown VCard properties in an extra column and load them when generating a new VCard (closes #118)
* upgrade to ez-vcard/0.9.3 (fixes sync error reported via Play Store)
* (re-)enable stale collection check, RetryHandler to retry idempotent CalDAV/CardDAV requests (hopefully fixes #225)
* always set FN/display name (take organization if no structured name is available) (hopefully fixes #227)
don't close httpClient while syncing