Added support for multiple loggers, changed default config to xml

Added support for multiple loggers.
log4php.xml is the default file for fetching properties as the ini  format does not support all features.
	new file:   ci_app/application/config/log4php.xml
	modified:   ci_app/application/helpers/log4php_helper.php
	modified:   ci_app/application/libraries/MY_Log.php