autoreload doesn't work on windows

[bdarnell]

os.execv on windows is broken on windows when arguments contain spaces, and now it can't be fixed for fear of breaking workarounds people have built on top of it: http://bugs.python.org/issue436259

It looks like subprocess does the right thing here, so autoreload should probably just use subprocess (and then exit from the parent process) on platforms that don't have a working unix-style os.execv.

[mmalecki]

@bdarnell
This seems to be a fix. Autoreload didn't work for me on Windows, because I have a white space in path to my executable.
It's also a bit tricky to test, but I will try to, if needed.
Review, please (or should I pull request?).