Updated WAV decoder.

[Ryan C. Gordon]

I've restructured the WAV decoder to allow for various formats, which will
allow for handling compressed WAVE types.

It's in CVS, but it may be broken; I don't have a soundcard available at
the moment to try it.  :)

I've put the start of an ADPCM compression handler in place, but not the
actual decompression code, so it should error out on the first read.
Hopefully uncompressed WAVs should still work.

--ryan.