Update trackcmp to use multiprocessing when comparing against CD image.

By splitting the image into track-sized chunks using seeking,
it's easy to run the compare concurrently.
Add ALACEncoder object to audiotools.encoders module.

It's mostly a thin wrapper around the internal ALAC encoding
functions to enable encoding ALAC frames without a PCMReader.
Minor low-level buffer type handling cleanups.

In theory, this makes it very easy to convert window_start/_end variables
to pointers which is a closer match to what the buffer window is
and would limit maximum buffer size to whatever a system's RAM could hold.

In practice, switching offsets from unsigned integers to pointers
incurs a significant speed penalty, likely due to all that 64-bit math.
Add missing binaries messages at open-time.

That is, if one's missing oggenc and oggdec needed for Vorbis support,
end users will now be told about it when opening .ogg files
and given hints on where to get the needed programs to open those files.
tracksplit bugfix

Minor adjustments to unit tests.