Some documentation for include/reddav.php and a new tpl-file.

Documented include/reddav.php a bit more to understand what it is
about to try to clean up some parts. Still a lot to be done here.
I put serveral @todo markers that need to be checked.

Moved some more HTML to template file.

Changed getETag() return format according to documentation. (ETag MUST
be surrounded by double-quotes)
Update SabreDAV from 1.8.9 to 1.8.10.