Added support for RIFF MP3 format (revised)

(This is a revised version of the pull request #256.)

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 used in Japan for a couple of years.