Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.

Use the SampleTraits template and let the compiler generate a special
case for S32 instead of reusing S24_P32.

This is easier and more efficient to loop on, because only two
variables get modified (src and dest).

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

For 32 bit dithering, reuse the 24 bit dithering code, but apply a 8
bit right shift first.

There is nothing 24 bit specific in the pcm_dither_24 struct.  Since
we want to reuse the struct for 32 bit dithering, let's drop the "_24"
suffix from the struct name.