#10 Replay gain support

[Pascal Wolff]

Support for the 'Replay gain' tags.
ReplayGain is the name of a technology created to
achieve the same playback volume of audio files.

See
http://doc.hydrogenaudio.org/wikis/hydrogenaudio/ReplayGain
for more information.
The original proposal is in
http://replaygain.hydrogenaudio.org/