don't forget demo_windows.py
update MANIFEST.in to include demo_windows.py and not include the demo
keys (they're in tests/ now). clean up the README to explain the demo
scripts better now, since there are so many of them. then fix up the
demo scripts to look in tests/ for the keys.
demo_windows.py doesn't need to call get_pty() (in fact, i think that's
blowing openssh's mind) and was executing the wrong command.