feature 1071: improve log system on API calls. The log can be activated by a
configuration setting. The log file can be changed also with a configuration
setting.
improvement: pwg.images.add API methods gets a cleaner way to log received
parameters.
git-svn-id: http://piwigo.org/svn/trunk@3662 68402e56-0260-453c-a942-63ccdbb3a9ee