ZarafaBridge: use restriction functions

Much cleaner than writing the restrictions out by hand.
SabreDAV: upgrade included version to 1.6.7

This is the latest 1.6.x release at the time of writing.
Unzip the SabreDAV-1.6.7 zipfile, copy the /lib/Sabre* files over.
SabreDAV: upgrade included version to 1.6.9

Latest at time of writing.
ChangeLog: add, and use to track changes.
README.md: add link to tarballs/zipfiles and to ChangeLog

The location of the tarballs/zipfiles on Github is not immediately
self-obvious, so provide a link at the top of the README. Also provide a
pointer to the ChangeLog, which keeps track of the code changes.
config.inc.php: increment version number