Using a different property to internally pass the sync-token around.

Issue #496.
Ensuring vobject tests are still running.
Automatically populate getctag with the sync-token, if available.
PHPUnit 4.2
Automatically exposing a 'ctag' if we have a token available that can take that role.

Fixes #496
Preparing for release.
Merge branch '2.0'

Conflicts:
	ChangeLog.md
	composer.json
	lib/CardDAV/Backend/PDO.php
	lib/DAV/CorePlugin.php
	tests/Sabre/CardDAV/Backend/AbstractPDOTest.php
Merge branch 'master' of github.com:fruux/sabre-dav