• Complete rewrite of the XML system. We now use our own sabre/xml,
  which has a much smarter XML Reader and Writer.
• BC Break: It's no longer possible to instantiate the Locks plugin without
  a locks backend. I'm not sure why this ever made sense.
• Simplified the Locking system and fixed a bug related to if tokens checking
  locks unrelated to the current request.
• FSExt Directory and File no longer do custom property storage. This
  functionality is already covered pretty well by the PropertyStorage plugin,
  so please switch.
• Renamed Sabre\CardDAV\UserAddressBooks to Sabre\CardDAV\AddressBookHome
  to be more consistent with CalendarHome as well as the CardDAV
  specification.
• Sabre\DAV\IExtendedCollection now receives a Sabre\DAV\MkCol object as
  its second argument, and no longer receives seperate properties and
  resourcetype arguments.
• MKCOL now integrates better with propertystorage plugins.
• The zip release ships with sabre/vobject 3.4.2,
  sabre/http 4.0.0-alpha1, sabre/event 2.0.1,
  sabre/uri 1.0.0 and sabre/xml 0.4.3.