ReplayGain support

[ngoomie]

Pragha is almost perfect for my uses, with the sole exception of the lack of ReplayGain support. As it stands now I often have to adjust playback volume when shuffling my whole library or listening to a mix of songs from different albums, which is a bit of a pain. ReplayGain would fix this. I don't personally care about being able to generate ReplayGain tags from within Pragha, just having it recognize and make use of them is all I need -- I already use an external tool for this part.