Added support for RIFF MP3 format

This pull request adds a support for the RIFF MP3 format.

RIFF MP3 (aka RMP or RMP3) is a variation of MP3 which is wrapped with RIFF and can have a RIFF INFO tag. It was created by some Japanese developers in late 90's and widely (but locally) used for a couple of years. I believe that most of you haven't heard of it because it has been hardly used outside Japan and almost completely superseded by ID3V2 tags.

Possibly, it looks useless to most people, but I need it.