This allows tests to run on appveyor (https://ci.appveyor.com/project/mikekap/buck/build/1.0.54). Also generates a buck.pex for windows (although I don't think it varies by platform to start with). There's also a few easy fixes for windows tests in here.
This is currently rebased on the python test selector implementation, since there's a few failing python tests on windows that need to be excluded.