SabreDAV 2.0: update composer.json, add author names

Sabre-Zarafa now requires SabreDAV 2.0.
README.md: note requirement of PHP 5.4 for SabreDAV 2.0
SabreDAV 2.0 fix: update PROPPATCH handler to use propPatch object

SabreDAV 2.0 introduces a new method of updating properties, where the handler
has to subscribe on the properties it can update. Update the code to use this
new way of doing things.
server.php: SabreDAV 2.0 fix: remove obsolete constant

SabreDAV 2.0 only has the \Sabre\DAV\Version::VERSION constant, not
::STABILITY. Remove the reference to the missing constant.