Convert audio formats to new ReplayGain interface.

It now operates like metadata with set/get/delete_replay_gain() methods.
This allows ReplayGain to be transferred during track conversion instead
of recalculated every time.

This also removes the use of external tools like mp3gain - which will
need to be internalized in order to apply ReplayGain to MP3 files.
Update utilities to use new ReplayGain interface.
Add a --remove-replay-gain option to tracktag
Fix tests for new ReplayGain interface.