Debian bug report #703342; PHP options 'open_basedir' / 'allow_url_fopen' are not handled properly

Link to bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=703342

Short summary: PHP has options to restrict access to files. DAViCal is not well prepared for that. It should be documented in the INSTALL notes which PATHS/FILES are affected and the code should be improved, so that there are no silent fails.