SabreDAV 1.8: namespace and file location changes

SabreDAV 1.8 is namespaced, in contrast to the older 1.6 line. Convert.
SabreDAV: remove bundled copy

SabreDAV 1.8 has a different directory layout from 1.6, which makes it
infeasible to include parts of the package, like we did with the 1.6 branch.
For instance the /vendor directory contains autoloader.php, which we need to
autoload the SabreDAV classes. A good reason to remove SabreDAV entirely and
require the end user to download and install it separately.
log4php: remove bundled copy

Bundling part of the package without proper licensing is against the project's
license, so remove it entirely from the tree. Provide instructions in README.md
on how to install log4php manually.
README.md: add fix/workaround for HTTP 412 errors

These can occur when the client specifies an ETag in the header, but SabreDAV
cannot match it to a card. That's because the backend does not calculate ETags
(since creating a VCard and hashing the contents is very expensive).
ZarafaFolder: always calculate CTag from all child cards

Previously, the CTag was calculated from the folder's last modification time if
available. It turns out that this time is not updated whenever a child is
edited in-place. To produce an accurate CTag, and thus to make clients update
properly, we need to look at the modification times on all the child cards.
This is unfortunate but seems inevitable.