Added user-configurable logging

Also see https://github.com/Pita/etherpad-lite/issues/916

log4js is already used by etherpad but it hasn't been configured in any of the server files yet. I added the 'etherpad_logging.json' file to the etherpad lite root directory - the default is a console appender so if the user doesn't change anything, the current behaviour doesn't change.