add logger

• logging mechanism is working fine with web2py (see http://web2py.com/books/default/chapter/29/04/the-core#Logging)
• It has been implemented successfully in limbra
• Create a dedicated logger for plugin_dbui → web2py.app.plugin_dbui
• Replace many debug statements by logger.debug(blabla) or logger.info(blabla)
• The debug keyword in the URL will be only used to load the debug version of JavaScript library
• ...