Make it possible to run tests on windows (with a little cygwin help)

Hi Gustavo,

This PR makes it actually possible to run tests in the native windows shell:

![image](https://cloud.githubusercontent.com/assets/1620265/4031208/ce4fdd60-2c64-11e4-974a-5a2c377c3786.png)

Making supervisor run on native windows is a lost cause. With a little extra finagling you can make it behave reasonably on Cygwin though.

Comments eagerly awaited :)
Please drop this line, and rename this file to syscall_windows_test.go. The go tool understands that convention.