blocking i/o not implemented for dsound

[rossb]

ReadStream? and WriteStream? are not implemented.

One possible way to implement this would be to have ReadStream? and WriteStream? operate directly on the directsound ring buffers. If they needed to pause, they could block on an event which gets woken up by an MMTimer using the same timing rules the callback routine uses, every time they get woken up they could try to proceed until they had completed their work.

[rossb]

See also #55 "Directsound callback scheduling could be improved" should probably consider fixing both at once.