CC-430: Audio normalization (Replaygain Support)

-Added support on server side

airtime_mvc/application/configs/constants.php

@@ -30,6 +30,7 @@ define('MDATA_KEY_BPM', 'bpm');
 define('MDATA_KEY_TRACKNUMBER', 'track_number');
 define('MDATA_KEY_CONDUCTOR', 'conductor');
 define('MDATA_KEY_LANGUAGE', 'language');
+define('MDATA_KEY_REPLAYGAIN', 'replay_gain');
 
 define('UI_MDATA_VALUE_FORMAT_FILE', 'File');
 define('UI_MDATA_VALUE_FORMAT_STREAM', 'live stream');