Store ReplayGain info for MP3s in ID3v2 instead of APE tags

It's been supported by mp3gain for years now and the docs say it's the preferred format in Picard: http://musicbrainz.org/doc/MusicBrainz_Picard/Documentation/Options#Tags

Also: http://wiki.hydrogenaudio.org/index.php?title=ReplayGain_1.0_specification#Metadata_format

Ultimately, it'd probably be best if you could set custom options from within the plugin settings in Picard.