Adds support for Replay Gain tags on MP4 files

 * Stores Replay Gain info in MP4 files using the format
   '---:com.apple.iTunes:replaygain_*' which is used by FB2K
Fixes a typo. Whoops.
Ugh, more typos.
Merge pull request #94 from daveisadork/m4a-replaygain

Adds support for Replay Gain tags on MP4 files