Use exit library on Windows

This is required on Windows 7 since calling process.exit
before the stdout/stderr streams drain seems to prevent the
parent process from receiving the correct output.
Lock exit version down
Move exit code to cli.coffee

This moves it out of the path of the specs and they can now just verify
the callback errors directly.
Remove unused process.exit spy
Expect error to be null instead of undefined
Merge pull request #146 from atom/ks-use-exit-lib-on-windows

Use exit library on Windows